Image display apparatus and image display methods

ABSTRACT

Provided are image display apparatus and image display methods capable of suitably making correction for variation of driving conditions due to an electric resistance of matrix wiring of a display panel by downsized hardware. The apparatus and methods involve a device of calculating voltage drop amounts caused by the resistance of row wires, for input image data, and a device of calculating image data with correction for the voltage drop amounts (corrected image data). An overflow processing circuit is provided so as to prevent overflow of the image data after the correction from an input range of a modulator, and the overflow is prevented by a gain. Since a gradation converter for changing a gradation conversion characteristic by a gain is provided in the stage preceding to the configuration for making the correction for influence of the voltage drop, it becomes feasible to cancel saturation characteristics of phosphors and to display images with high quality thereby.

This application is a division of U.S. application Ser. No. 10/299,668,filed Nov. 20, 2002, now U.S. Pat. No. 6,952,193.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to image display apparatus such astelevision receivers, display devices, etc. for receiving a televisionsignal or a display signal of a computer or the like to display animage, using a display panel equipped with a plurality of image formingdevices matrix-wired, and to image display methods.

2. Related Background Art

The conventional apparatus was provided with (n×m) image forming devicesarrayed in a matrix pattern as wired to m row wires and n column wiresand was configured to implement sequential scanning of the row wires andmodulation in the column direction, thereby driving a device group ofeach row simultaneously.

In the case of this driving, there occurred a voltage drop due to theelectrical resistance of wiring, in the row wiring, so as to pose aproblem of a defect due to decrease of the voltage placed between thetwo ends of the display devices.

In order to make correction for decrease of luminance due to the voltagedrop caused by the wiring resistance of the electrical connection wiresand others to the display devices as described above, Japanese PatentApplication Laid-Open No. 08-248920 discloses the technology about theimage display apparatus having a configuration of calculating correctiondata for the voltage drop by statistical operation and combiningelectron beam requirements with correction values.

The configuration of this image display apparatus described in theJapanese application is presented in FIG. 38. The configurationassociated with the correction for data in this apparatus is roughly asdescribed below. First, luminance data of one line in a digital imagesignal is added up at an adder 206 and a correction factor correspondingto the sum is read out of a memory 207. On the other hand, the digitalimage signal is subjected to serial-parallel conversion at a shiftregister 204, the resultant parallel signals are retained for apredetermined period of time at a latch 205, and then they are fed inpredetermining timing into multipliers 208 provided for the respectivecolumn wires. At each multiplier 208 the luminance data for each columnwire is multiplied by the correction data read out of the memory 207,the resultant corrected data is transferred to a modulation signalgenerator 209, modulation signals corresponding to the corrected dataare generated at the modulation signal generator 209, and an image isdisplayed on a display panel on the basis of the modulation signals.This apparatus is configured to perform statistical operation processinglike calculations of the sum and average for the digital image signal,e.g., the adding operation of luminance data of one line in the digitalimage signal at the adder 206, and make the correction based on thisvalue.

The conventional configuration as described above, however, required thelarge-scale hardware including the multipliers for the respective columnwires, the memory for providing the output of the correction data, theadder for supplying an address signal to the memory, and so on.

An object of the present invention is to provide image display apparatusand image display methods capable of making appropriate correction for aluminance variation and a chromaticity variation caused by a variationin driving conditions due to the electrical resistance of matrix wiringof the display panel by smaller-scale hardware.

SUMMARY OF THE INVENTION

In order to achieve the above object, an image display apparatusaccording to the present invention is an image display apparatuscomprising:

a plurality of image forming devices connected to a plurality of rowwires and column wires respectively and arranged in a matrix pattern;

scanning means connected to the row wires;

modulating means connected to the column wires;

gradation converting means for converting a gradation characteristic ofinput image data;

corrected image data calculating means for calculating corrected imagedata, which is image data after correction for influence of a voltagedrop caused by a resistance of the row wires and scanning means, for anoutput of the gradation converting means;

the modulating means outputting modulation signals to the column wires,with entry of the corrected image data,

wherein the gradation conversion characteristic is a characteristic ofmaking correction for a light emission characteristic of the imageforming devices in an absent state of the voltage drop.

Another image display apparatus of the present invention is an imagedisplay apparatus comprising:

a plurality of image forming devices connected to a plurality of rowwires and column wires respectively and arranged in a matrix pattern;

scanning means connected to the row wires;

modulating means connected to the column wires;

gradation converting means for converting a gradation characteristic ofinput image data;

corrected image data calculating means for calculating corrected imagedata, which is image data after correction for influence of a voltagedrop caused by a resistance of the row wires and scanning means, for anoutput of the gradation converting means; and

amplitude adjusting means having a function of multiplying data by afactor for adjustment of the amplitude of the corrected image data sothat the amplitude of the corrected image data matches an input range ofthe modulating means,

wherein the gradation converting means has a gradation conversioncharacteristic corresponding to the factor, and

wherein the modulating means outputs modulation signals to the columnwires, with entry of the corrected image data amplitude-adjusted by theamplitude adjusting means.

Still another image display apparatus of the present invention is animage display apparatus comprising:

a plurality of electron-emitting devices connected to a plurality of rowwires and column wires respectively and arranged in a matrix pattern;

scanning means connected to the row wires;

modulating means connected to the column wires;

gradation converting means for performing gradation conversion of inputimage data;

corrected image data calculating means for calculating corrected imagedata, which is image data after correction for influence of a voltagedrop caused by a resistance of the row wires and scanning means, for anoutput of the gradation converting means; and

amplitude adjusting means having a function of multiplying data by afactor for adjustment of the amplitude of the corrected image data sothat the amplitude of the corrected image data matches an input range ofthe modulating means, in which the gradation converting means has agradation conversion characteristic corresponding to the factor, and

in which the modulating means outputs modulation signals to the columnwires, with entry of the corrected image data amplitude-adjusted;

wherein with entry of nonzero, uniform image data common to all colors,a pulse width of an output pulse from the modulating means close to anoutput terminal of the scanning means becomes shorter than a pulse widthof an output pulse from the modulating means far from the outputterminal of the scanning means, and

saturation characteristics of phosphors dependent upon emitted chargeamounts of the electron-emitting devices are further canceled, so as toimplement such driving that any image data uniform and common to all thecolors is displayed at almost equal color temperature of white color,independent of emission luminance.

An image display method according to the present invention is an imagedisplay method by an image display apparatus comprising a plurality ofelectron-emitting devices connected to a plurality of row wires andcolumn wires one each respectively and arranged in a matrix pattern,scanning means connected to the row wires, modulating means connected tothe column wires, and phosphors opposed to the electron-emittingdevices, the image display method comprising:

a step of calculating emitted charge amount requirements with correctionfor light emission characteristics of the phosphors against emittedcharge amounts, according to image data as luminance requirements; and

a step of calculating corrected image data with correction for variationof the emitted charge amounts due to influence of a voltage drop causedby a resistance of the row wires and scanning means, according to theemitted charge amount requirements calculated,

wherein the modulating means applies pulse waveforms according to thecorrected image data thus calculated, to the column wires.

Another image display method of the present invention is an imagedisplay method by an image display apparatus comprising a plurality ofelectron-emitting devices connected to a plurality of row wires andcolumn wires one each respectively and arranged in a matrix pattern,scanning means connected to the row wires, and modulating meansconnected to the column wires, the image display method comprising:

a step of performing gradation conversion of canceling a light emissioncharacteristic of the electron-emitting devices in an absent state of avoltage drop caused by a resistance of the row wires and scanning means,for input image data; and

a step of making correction for influence of the voltage drop caused bythe resistance of the row wires and scanning means, for an output in thestep of performing the gradation conversion of canceling the lightemission characteristic,

wherein the modulating means applies pulse waveforms to the column wiresaccording to an output in the step of making the correction for theinfluence of the voltage drop.

Still another image display method of the present invention is an imagedisplay method by an image display apparatus comprising a plurality ofelectron-emitting devices connected to a plurality of row wires andcolumn wires one each respectively and arranged in a matrix pattern,scanning means connected to the row wires, and modulating meansconnected to the column wires, the image display method comprising:

a step of converting a gradation characteristic of input image data;

a step of making correction for influence of a voltage drop caused by aresistance of the row wires and scanning means, for an output in thestep of converting the gradation characteristic,

in which the modulating means applies pulse waveforms to the columnwires according to an output in the step of making the correction forthe influence of the voltage drop,

wherein the step of making the correction for the influence of thevoltage drop further comprises a step of adjusting the amplitude so thatthe output in the step of making the correction for the influence of thevoltage drop falls within an input range of the modulating means, and

wherein the step of converting the gradation characteristic is to selecta portion of a characteristic of canceling a light emissioncharacteristic of the electron-emitting devices in an absent state ofthe voltage drop caused by the resistance of the row wires and scanningmeans, according to an output in the step of adjusting the amplitude sothat the output falls in the input range of the modulating means.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing a schematic view of an image displayapparatus according to an embodiment of the present invention;

FIG. 2 is a diagram showing electrical connections of a display panel;

FIG. 3 is a graph showing the characteristics of surface conductionelectron-emitting devices;

FIG. 4 is a diagram showing a driving method of the display panel;

FIGS. 5A, 5B, and 5C are diagrams for explaining a degenerate model;

FIG. 6 is a graph showing voltage drop amounts calculated discretely;

FIG. 7 is a graph showing change amounts of emission currents calculateddiscretely;

FIGS. 8A, 8B, and 8C are diagrams for explaining another calculationmethod of correction data;

FIGS. 9A, 9B, and 9C are diagrams showing a calculation example ofcorrection data in the case where the size of image data is 128;

FIGS. 10A, 10B, and 10C are diagrams showing a calculation example ofcorrection data in the case where the size of image data is 192;

FIGS. 11A and 11B are diagrams for explaining an interpolation method ofcorrection data;

FIG. 12 is a block diagram showing a schematic configuration of an imagedisplay apparatus incorporating a gradation converter in a firstembodiment;

FIG. 13 is a block diagram showing a configuration of a scanning circuitin the image display apparatus;

FIG. 14 is a block diagram showing a configuration of an inverse γprocessor in the image display apparatus;

FIG. 15 is a block diagram showing a configuration of a data sequenceconverter in the image display apparatus;

FIG. 16 is a diagram showing an example of continuous frames;

FIG. 17 is a graph showing sizes of image data in continuous frames;

FIGS. 18A and 18B are graphs showing gains in continuous frames;

FIG. 19 is a diagram showing gradation characteristics in the case whereno correction is made for the voltage drop and there is no gradationconverter provided;

FIG. 20 is a diagram showing charge-luminance characteristics;

FIG. 21 is a diagram showing the property of canceling saturation ofphosphors in the case where no overflow process is carried out;

FIG. 22 is a diagram showing the relation between charge-luminancecharacteristics and gains;

FIG. 23 is a diagram showing the property of canceling saturation ofphosphors in the case where the gain is 1;

FIG. 24 is a diagram showing the property of canceling saturation ofphosphors in the case where the gain is ½;

FIG. 25 is a diagram showing the property of canceling saturation ofphosphors in the case where the gain is ¼;

FIG. 26 is a block diagram showing a configuration example 1 of thegradation converter;

FIG. 27 is a block diagram showing a configuration example 2 of thegradation converter;

FIGS. 28A, 28B, and 28C are diagrams to illustrate the structure andoperation of a modulator in the image display apparatus;

FIG. 29 is a timing chart of the modulator in the image displayapparatus;

FIG. 30 is a block diagram showing a configuration of a correction datacalculator in the image display apparatus;

FIGS. 31A and 31B are block diagrams showing a configuration of adiscrete correction data calculating unit in the image displayapparatus;

FIG. 32 is a block diagram showing a configuration of a correction datainterpolating unit;

FIG. 33 is a block diagram showing a configuration of a linearapproximation unit;

FIG. 34 is comprised of FIGS. 34A, 34B and 34C showing a timing chart ofthe image display apparatus;

FIG. 35 is a block diagram showing a schematic configuration of theimage display apparatus of a second embodiment;

FIG. 36 is a block diagram showing the configuration of the imagedisplay apparatus of the second embodiment with smaller-scale hardware;

FIG. 37 is a block diagram showing a configuration example of thegradation converter with smaller-scale hardware in the secondembodiment; and

FIG. 38 is a block diagram showing the configuration of the conventionalimage display apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will beillustratively described below in detail with reference to the drawings.It is, however, noted that the scope of the invention is by no meansintended to be limited to the dimensions, materials, shapes, relativelocations, etc. of the components described in the embodiments unlessotherwise stated in particular by specific description.

First Embodiment

(General Outline)

The display apparatus in which cold-cathode devices are arranged in apassive matrix, experiences the phenomenon in which the voltage dropoccurs because of currents flowing into the scanning wiring and thewiring resistance of the scanning wiring, so as to degrade the displayimage. Therefore, the image display apparatus according to theembodiment of the present invention is provided with a processingcircuit configured to make appropriate correction for the influence ofthe voltage drop in the scanning wiring on the display image andsubstantiate it in a relatively small circuit scale.

The correction circuit is a circuit that estimates the degradation ofthe display image caused by the voltage drop according to the inputimage data, determines the correction data to correct it, and makescorrection for the input image data.

Inventors conducted elaborate research on the image display apparatus ofthe type as described below, as the image display apparatusincorporating such a correction circuit.

The following will first describe the schematic of the display panel ofthe image display apparatus according to the embodiment of the presentinvention, the electrical connection of the display panel, thecharacteristics of surface conduction electron-emitting devices, adriving method of the display panel, the mechanism of the voltage dropdue to the electrical resistance of the scanning wiring, and acorrection method and apparatus for the influence of the voltage drop.

(Schematic of Image Display Apparatus)

FIG. 1 is a perspective view of the display panel used in the imagedisplay apparatus according to the present embodiment, in which part ofthe panel is cut away in order to show the interior structure. In thedrawing, numeral 1005 designates a rear plate, 1006 a side wall, and1007 a face plate; these members 1005 to 1007 form an airtight containerfor maintaining the interior of the display panel in vacuum.

A substrate 1001 is fixed to the rear plate 1005, and N×M cold-cathodedevices 1002 are formed on the substrate. The row wires (scanning wires)1003, column wires (modulation wires) 1004, and cold-cathode devices areconnected as shown in FIG. 2.

This wire connection structure is called a passive matrix.

A fluorescent film 1008 is formed on the bottom surface of the faceplate 1007. Since the image display apparatus of the present embodimentis a color display apparatus, phosphors of the three primary colors,red, green, and blue, which are used in the field of CRT, are separatelydeposited in the section of the fluorescent film 1008. The phosphors arearranged to form pixels located at positions irradiated with electrons(emission currents) emitted from the cold-cathode devices and formed ina matrix pattern corresponding to the respective pixels (sub-pixels) ofthe rear plate 1005.

A metal back 1009 is formed on the bottom surface of the fluorescentfilm 1008.

Hv represents a high-voltage terminal which is electrically connected tothe metal back 1009. When a high voltage is applied to the Hv terminal,a high voltage is placed between the rear plate 1005 and the face plate1007.

The present embodiment employs surface conduction electron-emittingdevices as the cold-cathode devices in the display panel as describedabove. It is also possible to use field emission type devices as thecold-cathode devices. The present invention can also be applied to theimage display apparatus in which self-emitting devices like EL devicesexcept for the cold-cathode devices are connected in matrix wiring anddriven.

(Characteristics of Surface Conduction Electron-Emitting Devices)

The surface conduction electron-emitting devices have the (emissioncurrent Ie) versus (device voltage Vf) characteristics and the (devicecurrent If) versus (device voltage Vf) characteristics as shown in FIG.3. Since the emission current Ie is extremely smaller than the devicecurrent If and it is difficult to illustrate them in an identical scale,the two graphs are illustrated in their respective scales different fromeach other.

The surface conduction electron-emitting devices have the threeproperties, described below, as to the emission current Ie.

First, the emission current Ie quickly increases as the voltage appliedto the device increases over a certain voltage (which will be called athreshold voltage Vth). On the other hand, the emission current Ie isalmost zero at voltages below the threshold voltage Vth.

Namely, the devices are nonlinear devices having the distinct thresholdvoltage Vth against the emission current Ie.

Second, the emission current Ie varies depending upon the voltage Vfapplied to the devices, and it is thus feasible to control the magnitudeof the emission current Ie, by varying the voltage Vf.

Third, the cold-cathode devices have the quick response property, and itis thus feasible to control the emission time of the emission current Ieby the duration of application of the voltage Vf.

By making use of the properties as described above, it becomes feasibleto suitably apply the surface conduction electron-emitting devices tothe display apparatus. For example, in the image display apparatus usingthe display panel shown in FIG. 1, display can be implemented on thebasis of sequential scanning of the display screen by making use of thefirst property. Namely, voltages above the threshold voltage Vthaccording to desired emission luminances are properly applied to devicesin driving, while a voltage below the threshold voltage Vth is appliedto devices in a non-selected state. By sequentially switching the drivendevices, display can be implemented on the basis of sequential scanningof the display screen.

By making use of the second property, emission luminances of thephosphors can be controlled by the voltages Vf applied to the devices,thereby enabling the image display.

By making use of the third property, light emission durations of thephosphors can be controlled by durations of application of the voltagesVf to the devices, thereby enabling the display of image.

In the image display apparatus according to the embodiment of thepresent invention, modulation was implemented using the above thirdproperty.

(Driving Method of Display Panel)

FIG. 4 is an example of voltages applied to voltage supply terminals ofscanning wires and modulation wires on the occasion of driving thedisplay panel of the image display apparatus according to the embodimentof the present invention.

Now let us suppose that a horizontal scanning period I stands for aduration of light emission from pixels in the ith row.

For light emission from the pixels in the ith row, the scanning wire ofthe ith row is brought into a selected state and a selection potentialVs is impressed to its voltage supply terminal D×i. The voltage supplyterminals D×k (k=1, 2, . . . N, provided that k ≠i) of the otherscanning wires are in a non-selected state and a non-selection potentialVns is impressed thereto.

In the present example, the selection potential Vs is set at −0.5V_(SEL), which is half of the voltage V_(SEL) illustrated in FIG. 3, andthe non-selection potential Vns is the GND potential.

Pulse width modulation signals with the voltage amplitude Vpwm aresupplied to the voltage supply terminals of the modulation wires.Conventionally, in the case of no correction being made, a pulse widthof a pulse width modulation signal supplied to the jth modulation wirewas determined according to the size of image data for the pixel of theith row and the jth column in the display image, and pulse widthmodulation signals according to sizes of image data of the respectivepixels were supplied to all the modulation wires.

In the embodiment of the present invention, as described later, in orderto make correction for the lowering of luminance due to the influence ofthe voltage drop, a pulse width of a pulse width modulation signalsupplied to the jth modulation wire is determined according to the sizeof image data for the pixel of the ith row and the jth column in thedisplay image and a correction amount therefor, and pulse widthmodulation signals thus determined are supplied to all the modulationwires.

In the present embodiment, the voltage Vpwm is set at +0.5 V_(SEL).

The surface conduction electron-emitting devices emit electrons duringapplication of the voltage V_(SEL) at the both ends of the devices, butemit no electron during application of any voltage smaller than Vth, asshown in FIG. 3.

There is also such a feature that the voltage Vth is larger than 0.5V_(SEL), as shown in FIG. 3.

For this reason, the surface conduction electron-emitting devicesconnected to the scanning wires under application of the non-selectionpotential Vns emit no electron.

Similarly, during a period in which the output of the pulse widthmodulator is the ground potential (which will be hereinafter referred toas a period of output “L”), the voltage applied to the two ends of eachsurface conduction electron-emitting device on the selected scanningwire is Vs, so that no electron is emitted therefrom.

Each surface conduction electron-emitting device on a scanning wire towhich the selection potential Vs is applied, emits electrons accordingto a period in which the output of the pulse width modulator is Vpwm(which will be hereinafter referred to as a period of output “H”). Whenelectrons are emitted, the corresponding phosphor described previouslyemits light according to the amount of the beam of emitted electrons, sothat light emission can be implemented at a luminance according to theduration of electron emission.

The image display apparatus according to the embodiment of the presentinvention displays an image by the line sequential scanning and pulsewidth modulation as described above.

(Voltage Drop in Scanning Wiring)

As described above, the essential problem of the image display apparatusis that the voltage drop in the scanning wiring of the display panelincreases the potential on the scanning wiring, so as to decrease thevoltage applied to each surface conduction electron-emitting device andthus lower the emission current from the surface conductionelectron-emitting device. The mechanism of this voltage drop will bedescribed below.

Although it may differ depending upon design specifications andproduction processes of the surface conduction electron-emittingdevices, the device current from one of the surface conductionelectron-emitting devices is approximately several hundred μA duringapplication of the voltage V_(SEL).

For this reason, in the case where only one pixel on a scanning lineselected in a certain horizontal scanning period is made to emit lightwhile the other pixels are kept off, the device current flowing from themodulation wire into the scanning wire of the selection row is just thecurrent of one pixel (i.e., aforementioned several hundred μA), and thevoltage drop is almost zero, so as to cause no decrease of emissionluminance.

However, in the case where all the pixels in a selected row are made toemit light in a certain horizontal scanning period, currents of all thepixels flow from all the modulation wires into the scanning wire in theselected state, so that the total current becomes several hundred mA toseveral A, which used to cause the voltage drop on the scanning wirebecause of the wiring resistance of the scanning wire.

When the voltage drop occurs on the scanning wire, voltages applied tothe two ends of the surface conduction electron-emitting devices arelowered. This resulted in decreasing the emission currents from thesurface conduction electron-emitting devices, so as to decrease theemission luminances.

To make matters more complex, the voltage drop has the nature that themagnitude thereof also varies even during one horizontal scanning periodbecause of the modulation based on the pulse width modulation.

Let us consider a case where the pulse width modulation signals suppliedto the respective columns are those with pulse widths depending uponsizes of data and with a synchronized rise, for the input data as shownin FIG. 4. In this case, it is general, though it depends upon the inputimage data, that within one horizontal scanning period, the number oflighted pixels increases toward immediately after the rise of pulses andthe number of lighted pixels decreases with time within one horizontalscanning period, because the pixels become unlighted in order from thelowest luminance part after the rise.

Accordingly, the magnitude of the voltage drop on the scanning wire isthe greatest in the initial part of one horizontal scanning period andtends to decrease gradually thereafter.

Since outputs of pulse width modulation signals vary at time intervalscorresponding to respective gradation levels of modulation, the voltagedrop also changes with time at time intervals equivalent to therespective gradation levels of the pulse width modulation signals.

The above described the voltage drop on the scanning wiring.

(Calculation Method of Voltage Drop)

The following will detail a method of making correction for theinfluence of the voltage drop.

The first step necessary for determining correction amounts for reducingthe influence of the voltage drop is to develop hardware capable ofestimating the magnitude of the voltage drop and the temporal changethereof in real time.

However, the display panel of the image display apparatus as in thepresent invention generally has several thousand modulation wires and itis very difficult to calculate voltage drops at intersections betweenall the modulation wires and a scanning wire. Therefore, it is not sopractical to fabricate hardware for calculating them in real time.

Thus the voltage drop amounts are determined by grouping the devicesinto blocks as to positions on the same row and also grouping image datainto blocks as to amplitudes thereof.

This grouping into blocks is based on the following features of thevoltage drops.

i) At a certain point of time in one horizontal scanning period, thevoltage drops appearing on the scanning wire are spatially continuousamounts on the scanning wire and are represented by a very smooth curve.

ii) The magnitudes of voltage drops, which differ depending upon thedisplay image, vary at time intervals equivalent to the respectivegradation levels of the pulse width modulation; schematically, they arelarge at the rise of pulses, and they gradually decrease with time orare maintained.

Namely, in the driving method as shown in FIG. 4, the magnitudes ofvoltage drops never increase within one horizontal scanning period.

Specifically, the time change of voltage drops was schematicallyestimated by calculating the voltage drops at a plurality of times onthe basis of a degenerate model described below.

(Calculation of Voltage Drops by Degenerate Model)

FIG. 5A is a diagram for explaining blocks and nodes employed indegeneration.

For simplicity of illustration, FIG. 5A shows only a selected scanningwire, modulation wires, and surface conduction electron-emitting devicescoupled at intersections between the wires.

It is assumed herein that the present time is a certain time in onehorizontal scanning period and lighting conditions of the respectivepixels on the selected scanning wire (i.e., whether the output of themodulator is “H” or “L” for each device) are known.

In this lighting state, device currents flowing from the respectivemodulation wires into the selected scanning wire are defined as Ifi(i=1, 2, . . . . N, where i represents a column number).

As shown in the same figure, a block is defined as one group including nmodulation wires, a portion of the selected scanning wire intersectingtherewith, and surface conduction electron-emitting devices placed atintersections between them. In the present example, the part of interestwas divided into four blocks by grouping into blocks.

Positions denoted by nodes are defined at the boundary positions of therespective blocks. The nodes are horizontal positions (reference points)for discretely calculating the voltage drop amounts appearing on thescanning wire in the degenerate model.

In the present example five nodes, node 0 to node 4, are set at theboundary positions of the blocks.

FIG. 5B is a diagram for explaining the degenerate model.

In the degenerate model n modulation wires included in one block in FIG.5A are degenerated into one, and one degenerate modulation wire isconnected so as to be located in the center of each block of thescanning wire.

It is assumed that a current source is connected to a modulation wire ofeach degenerate block and summations IF0-IF3 of currents in therespective blocks flow from the respective current sources into themodulation wires.

Namely, IFj (j=0, 1, . . . 3) represents the electric current expressedby the equation below.

$\begin{matrix}{{IFj} = {\sum\limits_{i = {{j \times n} + 1}}^{{({j + 1})} \times n}{Ifi}}} & \left( {{Eq}\mspace{20mu} 1} \right)\end{matrix}$

The potential at the both ends of the scanning wire is Vs in the exampleof FIG. 5A, whereas it is the GND potential in FIG. 5B. The reason forit is that in the degenerate model the electric currents flowing fromthe modulation wires into the selected scanning wire are modeled by theabove current sources and thus the voltage drop amounts of therespective portions on the scanning wire can be calculated bydetermining voltages (potential differences) of the respective portionswith respect to the power supply part at the reference (GND) potential.

Namely, the ground potential is defined as a reference potential incalculation of voltage drops.

The reason why the surface conduction electron-emitting devices areomitted is that, as far as the selected scanning wire is concerned, thepresence or absence of the surface conduction electron-emitting devicesmakes no change in the appearing voltage drops themselves if equivalentelectric currents flow from the column wires. Therefore, the surfaceconduction electron-emitting devices are omitted herein while the valuesof currents flowing from the current sources of the respective blocksare set at the current values (Eq 1) equal to the summations of thedevice currents in the respective blocks.

A wiring resistance of the scanning wire of each block is defined as ntimes a wiring resistance r of the scanning wire in one interval (oneinterval herein refers to a zone between an intersection of the scanningwire with a certain column wire and an intersection of the scanning wirewith a column wire adjacent thereto and it is also assumed in thepresent example that the wiring resistance of the scanning wire isuniform throughout each interval).

In the degenerate model described above, the voltage drop amounts DV0 toDV4 appearing at the respective nodes on the scanning wire can bereadily calculated by equations below in the form of sum of products.DV0=a00×IF0+a01×IF1+a02×IF2+a03×IF3DV1=a10×IF0+a11×IF1+a12×IF2+a13×IF3DV2=a20×IF0+a21×IF1+a22×IF2+a23×IF3DV3=a30×IF0+a31×IF1+a32×IF2+a33×IF3DV4=a40×IF0+a41×IF1+a42×IF2+a43×IF3

Namely, the following equation holds

$\begin{matrix}{{DVi} = {\sum\limits_{j = 0}^{3}{{aij} \times {IFj}}}} & \left( {{Eq}\mspace{20mu} 2} \right)\end{matrix}$

(i=0, 1, 2, 3, or 4)

In the above equation, aij represents a voltage appearing at the ithnode when a unit current is injected into only the jth block in thedegenerate model (which will be defined hereinafter as aij).

The above aij is derived by the Kirchhoff's law and the result of thecalculation thereof once performed can be stored in the form of a table.

Furthermore, the approximation as expressed by Eq 4 below is effectedfor the summation currents IF0-IF3 of the respective blocks defined inEq 1.

$\begin{matrix}{{IFj} = {{\sum\limits_{i = {{j \times n} + 1}}^{{({j + 1})} \times n}{Ifi}} = {{IFS} \times {\sum\limits_{i = {{j \times n} + 1}}^{{({j + 1})} \times n}{{Count}\mspace{14mu} i}}}}} & \left( {{Eq}\mspace{20mu} 4} \right)\end{matrix}$

In the above equation, Count i represents a variable that takes 1 whenthe ith pixel on the selected scanning line is in a lighted state andthat takes zero when the ith pixel is in an unlighted state.

IFS indicates a quantity obtained by multiplying a device current IFflowing during application of the voltage V_(SEL) at the two ends of oneof the surface conduction electron-emitting devices, by a factor αtaking a value between 0 and 1.

Namely, it is defined as follows.IFS=α×IF  (Eq 5)

Eq 4 is based on the assumption that a device current proportional tothe number of lighting devices in each block flows from a column wire ofthe block into the selected scanning wire. The following is the reasonwhy the product of the device current IF of one device and thecoefficient α is defined as the device current IFS of one device. Inorder to calculate a voltage drop amount, it is originally necessary torepeatedly calculate a voltage increase of the scanning wire due to avoltage drop and a decrease amount of the device current caused thereby,but it is not practical to perform this convergence calculation byhardware. Therefore, the present invention employs approximate αIF as aconvergence value of IF. Specifically, the factor is determined asfollows: preliminarily estimated are a decrease rate of IF at themaximum voltage drop amount (with all devices turn-on) (=α1) and adecrease rate of IF at the minimum voltage drop amount (minimum=0)(=α2); and it is then determined as an average of α1 and α2 or as0.8×α1.

FIG. 5C is an example of the result of calculation of voltage dropamounts DV0-DV4 at the respective nodes by the degenerate model, in acertain lighting state.

Since the voltage drop is given by a very smooth curve, the voltage dropbetween nodes is assumed to take approximate values as indicated by adotted line in the drawing.

As described above, the use of the present degenerate model permits thevoltage drop to be calculated at the positions of the nodes and at anydesired point of time for the input image data.

As described above, the voltage drop amounts in a certain lighting stateare simply calculated using the degenerate model.

The voltage drop appearing on the selected scanning wire varies withtime in one horizontal scanning period, and this temporal change wasestimated in such a manner that lighting states were determined atseveral points of time in one horizontal scanning period, as describedpreviously, and voltage drops in the respective lighting states weredetermined using the degenerate model.

The number of lighting devices in each block at a certain point of timein one horizontal scanning period can be readily determined by makingreference to the image data of each block.

Let us suppose here that the bit count of input data into the pulsewidth modulator is 8 bits as an example and the pulse width modulatorprovides outputs of pulse widths according to sizes of the input data.

Namely, it is assumed that with the input data of 0, the output is “L”;with the input data of 255, the output is “H” throughout one horizontalscanning period; with the input data of 128, the output is “H” duringthe first half of one horizontal scanning period and “L” during thesecond half.

In this case, the number of lighting devices at the time of the start ofpulse width modulation signals (at the time of the rise in the exampleof the modulation signals in the present example) can be readilydetected by counting the input data into the pulse width modulatorgreater than 0.

Likewise, the number of lighting devices at the time of the middle ofone horizontal scanning period can be readily detected by counting theinput data into the pulse width modulator greater than 128.

In this way the number of lighting devices at an arbitrary time can bereadily calculated by comparing the image data with a certain thresholdin a comparator and counting true outputs of the comparator.

For simplification of the description hereinafter, let us define a timeslot as a time quantity.

Namely, the time slot indicates a time from the rise of the pulse widthmodulation signals in one horizontal scanning period, and the timeslot=0 is defined as one indicating a time immediately after the time ofthe start of a pulse width modulation signal.

The time slot=64 is defined as a time elapsed for a duration of sixtyfour gradation levels from the time of the start of a pulse widthmodulation signal.

The present example provided the example in which the pulse widths-weremodulated with respect to the reference at the time of the rise. It isneedless to mention that the present invention can also be similarlyapplied to the case where the pulse widths are modulated with respect tothe reference at a time of a fall of pulses, though the advancingdirection of the time axis becomes reverse to the advancing direction ofthe time slot.

(Calculation of Correction Data from Voltage Drop Amounts)

As described above, we succeeded in approximately and discretelycalculating the temporal change of the voltage drop during onehorizontal scanning period by the iterative calculation using thedegenerate model.

FIG. 6 is an example in which the temporal change of the voltage drop onthe scanning wire was calculated on the basis of the iterativecalculation of voltage drop for certain image data. (It is noted thatthe voltage drop and the temporal change thereof presented herein arejust an example for certain image data and it is a matter of course thatthe voltage drop for another image data demonstrates another change.)

FIG. 6 shows the result of calculation in which the voltage drop amountsat the respective times were discretely calculated by applying thedegenerate model to each of four time points of time slots=0, 64, 128,and 192.

In FIG. 6 the voltage drop amounts at the respective nodes are connectedby dotted lines, and it is noted that the dotted lines are presented fora better look of illustration. In fact, the voltage drop amountscalculated by the degenerate model were discretely calculated at thepositions of each node as indicated by □, ∘, ●, and Δ.

Inventors conducted research on a method of calculating the correctiondata for correction for image data from the voltage drop amounts, as thenext stage after the implementation of calculation of the magnitude andtemporal change of voltage drop.

FIG. 7 is a graph of estimated amounts of emission current emitted fromthe surface conduction electron-emitting devices in the lighted stateunder the condition that the voltage drop shown in FIG. 6 occurred onthe selected scanning wire.

The vertical axis indicates an amount of emission current at each timeand at each position in percentage with respect to 100% as an amount ofemission current emitted in the case of no voltage drop, and thehorizontal axis the horizontal position.

As shown in FIG. 7, let us define emission currents at the horizontalposition of node 2 (reference point) as follows:

Ie0: emission current at the time slot=0;

Ie1: emission current at the time slot=64;

Ie2: emission current at the time slot=128;

Ie3: emission current at the time slot=192.

FIG. 7 shows the result of calculation from the graphs of the voltagedrop amounts of FIG. 6 and the “drive voltage-emission current” of FIG.3. Specifically, it is a mechanical plot of values of emission currentduring application of voltages obtained by subtracting the voltage dropamounts from the voltage V_(SEL).

Accordingly, FIG. 7 consistently shows the currents emitted from thesurface conduction electron-emitting devices in the lighted state, andthe surface conduction electron-emitting devices in the unlighted stateemit no current.

The following will describe a method of calculating the correction datafor correction for image data from the voltage-drop amounts.

(Correction Data Calculating Method)

FIGS. 8A, 8B, and 8C are diagrams for explaining a method of calculatingcorrection data of a voltage drop amount from the temporal change ofemission current shown in FIG. 7. These figures show an example of thecalculation of the correction data for the image data in the size of 64.

The quantity of emitted light with a luminance is nothing but the amountof emitted charge which is obtained by integrating the emission currentduring an emission current pulse over time. For considering variation ofluminance due to the voltage drop, therefore, the descriptionhereinafter will be given on the basis of the amount of emitted charge.

Let IE be an emission current in the case of no influence of the voltagedrop, and Δt be a time equivalent to one gradation level of the pulsewidth modulation. Then an emitted charge amount Q0 of charge to beemitted by an emission current pulse for the image data of 64 can beexpressed as follows by multiplying the amplitude IE of the emissioncurrent pulse by the pulse width (64×Δt).Q0=IE×64×Δt  (Eq 6)

In practice, however, there occurs the phenomenon of decrease ofemission current due to the voltage drop on the scanning wire.

The emitted charge amount with the emission current pulse taking accountof the influence of the voltage drop can be calculated approximately asfollows. Namely, let Ie0 and Ie1 be emission currents in the respectivetime slots=0 and 64 at the node 2, and let us assume that the emissioncurrent between 0 and 64 is approximated so as to linearly changebetween Ie0 and Ie1. Then the emitted charge amount Q1 during thisperiod is given by the area of the trapezoid shown in FIG. 8B.

Namely, it can be calculated as follows.Q1=(Ie0+Ie1)×64×Δt×0.5  (Eq 7)

Let us then suppose that, as shown in FIG. 8C, the influence of thevoltage drop was successfully eliminated when the pulse width wasextended by DC1 in order to make correction for the decrease of theemission current due to the voltage drop.

It is considered that the emission current amount in each time slotvaries when the pulse width is extended for the correction for thevoltage drop. For simplification, however, it is assumed herein that theemission current is Ie0 in the time slot=0 and the emission current Ie1in the time slot=(64+DC1), as shown in FIG. 8C.

An approximation is also made so that the emission current between thetime slot 0 and the time slot (64+DC1) takes values on a straight lineconnecting the emission currents at the two points.

Then, an emitted charge amount Q2 with an emission current pulse afterthe correction can be calculated as follows.Q2=(Ie0+Ie1)×(64+DC1)×Δt×0.5  (Eq 8)

By equating this to aforementioned Q0, we obtain the following.IE×64×Δt=(Ie0+Ie1)×(64+DC1)×Δt×0.5

By solving this with respect to DC1, we obtain the following.DC1=((2×IE−Ie0−Ie1)/(Ie0+Ie1))×64  (Eq 9)

In this way, the correction data was calculated for the case of theimage data being 64.

Namely, for the image data in the size of 64 at the position of the node2, the correction amount CData to be added can be given by CData=DC1, asrepresented by Eq 9.

Likewise, a correction amount can be determined for each of two periods,as shown in FIGS. 9A to 9C, for the image data in the size of 128, and acorrection amount can be determined for each of three periods, as shownin FIGS. 10A to 10C, for the image data in the size of 192.

Since the influence of the voltage drop on the emission current is, ofcourse, null with the pulse width of 0, the correction data was set as 0and the correction data CData to be added to the image data was also setas 0.

The reason why the correction data was calculated for discrete imagedata like 0, 64, 128, and 192 as described above, is that it can reducethe calculation amount.

FIG. 11A shows an example of discrete correction data for certain inputimage data, which was obtained by the present method. In the same figurethe horizontal axis represents the horizontal display position and thepositions of the respective nodes are illustrated. The vertical axisrepresents the size of correction data.

The discrete correction data was calculated at the positions of thenodes and for the sizes of the image data Data (image data referencevalues=0, 64, 128, and 192) as indicated by □, ∘, ●, and Δ in thedrawing.

(Interpolation Method of Discrete Correction Data)

The correction data discretely calculated is discrete data at thepositions of the respective nodes, and does not give correction data atan arbitrary horizontal position (column wiring number). At the sametime as it, the correction data is data for image data in severalpredetermined sizes of reference values of image data at each nodeposition, but does not give correction data according to the actualsizes of image data.

Then Inventors calculated the correction data fitting sizes of inputimage data in the respective column wires by interpolation of thediscretely calculated correction data.

FIG. 11B is a diagram showing a method of calculating the correctiondata for the image data Data at x located between the node n and thenode (n+1).

As a premise, the correction data has already discretely been calculatedat the positions Xn and Xn+1 of the node n and the node n+1.

It is also assumed that the input image data Data takes values betweenthe image data reference values Dk and Dk+1.

Letting CData [k][n] be the discrete correction data for the referencevalue of the kth image data at the node n, the correction data CA forthe pulse width Dk at the position x can be calculated as follows bylinear approximation, using the values of CData [k] [n] and CData [k][n+1].

Namely, the correction data CA is given as follows.

$\begin{matrix}{{CA} = \frac{{\left( {{Xn} + 1 - x} \right) \times {{{CData}\lbrack k\rbrack}\lbrack n\rbrack}} + {\left( {x - {Xn}} \right) \times {{{CData}\lbrack k\rbrack}\left\lbrack {n + 1} \right\rbrack}}}{{Xn} + 1 - {Xn}}} & \left( {{Eq}\mspace{20mu} 17} \right)\end{matrix}$

In this equation, Xn and Xn+1 indicate the horizontal display positionsof the respective nodes n and (n+1), which are constants settled in thedetermination of the aforementioned blocks.

The correction data CB for the image data Dk+1 at the position x can becalculated as follows.

Namely, the correction data CB is given by the following equation.

$\begin{matrix}{{CB} = \frac{{\left( {{Xn} + 1 - x} \right) \times {{{CData}\left\lbrack {k + 1} \right\rbrack}\lbrack n\rbrack}} + {\left( {x - {Xn}} \right) \times {{{CData}\left\lbrack {k + 1} \right\rbrack}\left\lbrack {n + 1} \right\rbrack}}}{{Xn} + 1 - {Xn}}} & \left( {{Eq}\mspace{20mu} 18} \right)\end{matrix}$

By linear approximation with the correction data CA and CB, thecorrection data CD for the image data Data at the position x can becalculated as follows.

Namely, the correction data CD is given by the following equation.

$\begin{matrix}{{CD} = \frac{{{CA} \times \left( {{Dk} + 1 - {Data}} \right)} + {{CB} \times \left( {{Data} - {Dk}} \right)}}{{Dk} + 1 - {Dk}}} & \left( {{Eq}\mspace{20mu} 19} \right)\end{matrix}$

As described above, the correction data matching the actual positionsand sizes of image data can be readily calculated from the discretecorrection data by the method as described by Eq 17 to Eq 19.

The dashed lines connecting the nodes in FIG. 11A are the results ofinterpolation from the discrete correction data by the abovecalculation. As seen from the figure, the voltage drop correction methodof the present invention results in yielding the same correction datafor all the positions x (also including the correction data of 0, ofcourse), because there occurs no-voltage drop in the case of the imagedata of 0; and it results in obtaining the correction data in agently-curved distribution for the positions x, i.e., the horizontaldirection of the screen in the case of the same image data being notequal to 0. It is, however, noted that, in the case of the scanninglines being directed in the perpendicular direction in the screen, thecorrection data is one in a gently curved distribution in theperpendicular direction in the screen.

By correcting the image data by adding the correction data thuscalculated, to the image data and effecting the pulse width modulationaccording to the image data after the correction (referred to ascorrected image data), it becomes feasible to reduce the influence ofthe voltage drop on the display image, which was the problem heretofore,and thereby improve the quality of image.

There is also the excellent advantage that the hardware for thecorrection, which was the problem heretofore, can be constructed in verysmall scale, because the calculation amount can be decreased byintroduction of the approximations including the degeneracy as describedabove.

(Description of Entire System and Functions of Respective Portions)

The following will describe the hardware of the image display apparatusincorporating the correction data calculator.

FIG. 12 is a block diagram schematically showing a circuit configurationof the apparatus. In the drawing, numeral 1 designates the display panelof FIG. 1; Dx1-DxM and Dx1′-DxM′ voltage supply terminals of thescanning wires of the display panel; Dy1-DyN voltage supply terminals ofthe modulation wires of the display panel; Hv a high-voltage supplyterminal for placing an acceleration voltage between the face plate andthe rear plate; Va a high voltage power source; 2 and 2′ scanningcircuits; 3 a synchronizing signal separator; 4 a timing generator; 7 aconverter for converting a YPbPr signal separated by the synchronizingsignal separator 3, into RGB signals; 23 a selector for implementingchangeover between a television video signal and a computer videosignal; 17 an inverse γ processing unit; 5 a shift register for one lineof image data; 6 a latch for one line of image data; 8 a pulse widthmodulator for outputting modulation signals into the modulation wires ofthe display panel; 12 an adder; 14 a correction data calculator; 20 amaximum value detector; 21 a gain calculator; 200 a gradation converter.

Since the gradation converter 200 will be described later, thedescription below will be given excluding the gradation converter 200.

In the same drawing, R, G, and B represent parallel RGB input videodata; Ra, Ga, and Ba parallel RGB video data subjected to inverse γconversion processing described hereinafter; Data image data obtained byparallel-serial conversion at the data sequence converter; CD thecorrection data calculated by the correction data calculator; Dout thecorrected image data (adjusted image data) obtained by adding thecorrection data to the image data at the adder.

(Synchronizing Separator and Selector)

The image display apparatus of the present embodiment can displaytelevision signals of NTSC, PAL, SECAM, HDTV, etc., and VGA and the likeas computer output.

A video signal of the HDTV system is first fed into the synchronizingseparator 3, in which synchronizing signals Vsync, Hsync are separatedtherefrom and are supplied into the timing generator. The video signalsynchronously separated is supplied into the RGB converter. Inside theRGB converter there are provided a low-pass filter, an A/D converter,etc., not shown, in addition to the converter from YPbPr into RGB. TheRGB converter converts the YPbPr signal into digital RGB signals andsupplies the RGB signals into the selector 23.

A video signal of VGA or the like outputted from a computer is subjectedto A/D conversion at an unrepresented A/D converter and the resultantdigital signal is supplied into the selector 23.

The selector 23 properly selects either of the television signal and thecomputer signal and outputs the signal selected based on which videosignal the user desires to display.

(Timing Generator)

The timing generator incorporates a PLL circuit and is a circuit forgenerating timing signals compatible with various video formats and thusgenerating operation timing signals of the respective units.

The timing signals generated by the timing generator 4 include Tsft forcontrol over the operation timing of the shift register 5, a controlsignal Dataload for latching of data from the shift register into thelatch 6, a pulse width modulation start signal Pwmstart of the modulator8, a clock Pwmclk for the pulse width modulation, Tscan for control overthe operation of the scanning circuits 2 and 2′, and so on.

(Scanning Circuits)

As shown in FIG. 13, the scanning circuits 2 and 2′ are circuits forsupplying a selected voltage Vs or a non-selected voltage Vns into theconnection terminals Dx1-DxM in order to implement sequential scanningof the display panel by one row per horizontal scanning period.

The scanning circuits 2 and 2′ are circuits for implementing scanningwhile sequentially switching the scanning wire selected in eachhorizontal period one from another in synchronism with the timing signalTscan from the timing generator 4.

Tscan is a timing signal group made from the vertical synchronizingsignal and horizontal synchronizing signal, for example.

Each of the scanning circuits 2 and 2′ is comprised of M switches, ashift register, and others as shown in FIG. 13. These switches arepreferably constructed of transistors or FETs.

In order to decrease the voltage drop on the scanning wires, thescanning circuits are preferably coupled to the two ends of the scanningwires of the display panel, as shown in FIG. 12, and are configured todrive the panel from the two ends.

On the other hand, the embodiment of the present invention is alsoeffective in the case where the scanning circuits are not coupled to theboth ends of the scanning wires, and can be applied thereto by onlymodifying the parameters in Eq 2.

(Inverse γ Processor)

CRT has the approximately 2.2th power emission characteristic(hereinafter referred to as an inverse γ characteristic) against input.

The input video signal takes account of this characteristic of CRT, andis generally converted according to the 0.45th power γ characteristic soas to achieve a linear emission characteristic in display on the CRT.

On the other hand, the display panel of the image display apparatusaccording to the embodiment of the present invention has anapproximately linear emission characteristic against applied time in thecase of the modulation being made by applied durations of the drivingvoltage, and it is thus necessary to convert the input video signal onthe basis of the inverse γ characteristic (hereinafter referred to asinverse γ conversion).

The inverse γ processor shown in FIG. 12 is a block for implementing theinverse γ conversion of the input video signal.

The inverse γ processor of the present embodiment is comprised ofmemories for implementing the above inverse γ conversion process.

On the assumption the bit count of the video signals R, G, and B is 8bits and that the bit count of the video signals Ra; Ga, and Ba asoutputs of the inverse γ processor is also 8 bits, the inverse γprocessor is comprised of memories of 8-bit address and 8-bit data forthe respective colors (FIG. 14).

(Data Sequence Converter)

The data-sequence converter 9 is a circuit of performing parallel-serialconversion of the parallel RGB video signals Ra, Ga, and Ba so as to fitthe pixel array of the display panel. The data sequence converter 9 iscomprised of FIFO (First In First Out) memories 2021R, 2021G, and 2021Bfor the respective colors R, G, and B and a selector 2022, as shown inFIG. 15.

Although not shown in the same figure, each FIFO memory is provided withtwo memories of the word equal to the number of horizontal pixels, onefor odd lines and one for even lines. With entry of video data of an oddrow, the data is written into the FIFO memories for odd lines, while theimage data stored during one preceding horizontal scanning period isread out of the FIFO memories for even lines. With entry of video dataof an even row, the data is written into the FIFO memories for evenlines, while the image data stored during one preceding horizontalperiod is read out of the FIFO memories for odd lines.

Data read out of the FIFO memory is subjected to parallel-serialconversion according to the pixel array of the display panel at theselector, and serial GB image data SData is outputted therefrom. Thefurther details will not be given herein, but the operation is performedbased on the timing control signal from the timing generator 4.

(Delay circuit 19)

The image data SData after the rearrangement at the data sequenceconverter is fed into the correction data calculator and into the delaycircuit 19. A correction data interpolation unit of the correction datacalculator, described later, makes reference to values of the horizontalposition information x from the timing control circuit and the imagedata SData to calculate correction data CD matching them.

The delay circuit 19 is a means provided for absorbing the timenecessary for the calculation of the correction data (the aforementionedinterpolation process of correction data) and configured to give such adelay as to correctly add corresponding correction data to image data onthe occasion of adding the correction data to the image data at theadder. This means can be constructed of a flip-flop circuit.

(Adder 12)

The adder 12 is a unit of adding the correction data CD from thecorrection data calculator to the image data Data. The addition makesthe correction for the image data Data and the corrected image data Doutis transferred to the maximum value detector and to the multiplier.

The bit count of the corrected image data being the output of the adderis preferably settled so as to cause no overflow in the addition of thecorrection data to the image data.

More specifically, let us suppose that the image data Data has the datawidth of 8 bits and the maximum thereof is 255 and that the correctiondata CD has the data width of 7 bits and the maximum thereof is 120.

In this case, the maximum of the addition result becomes 255+120=375.

Under the above condition, the corrected image data Dout being theoutput of the adder is preferably output with the output bit width of 9bits in order to prevent the overflow.

(Overflow Process)

In the embodiment of the present invention, the correction isimplemented by the corrected image data resulting from the addition ofthe calculated correction data to the image data, as describedpreviously.

Now suppose that the bit count of the modulator is 8 bits and the bitcount of the corrected image data Dout being the output of the adder is10 bits.

Then overflow will occur if the corrected image data is supplied to theinput of the modulator as it is.

It is thus necessary to adjust the amplitude of the corrected imagedata, prior to the supply into the modulator.

A potential configuration for preventing the overflow is topreliminarily estimate a maximum of the corrected image data upon entryof maximum input image data in an all-devices-turn-on pattern ((R, G,B)=(FFh, FFh, FFh) in the case of the bit count of the image data being8 bits) and multiply the corrected image data by a gain so as to keep itwithin the input range of the modulator.

This method will be referred to hereinafter as a fixed gain method.

The fixed gain method does not cause the overflow, but is configured tomultiply the corrected image data for an image with a low averageluminance by the small gain, though it can be displayed with a greatergain. Therefore, the display image can be dark in luminance.

In contrast to it, another potential method for preventing the overflowis a method of detecting a maximum of the corrected image data in everyframe, calculating a gain so as to keep the maximum within the inputrange of the modulator, and multiply the corrected image data by thegain.

This method will be referred to hereinafter as an adaptive gain method.

The adaptive gain method necessitates the maximum value detector 20 fordetecting the maximum MAX of the corrected image data Dout in everyframe, the gain calculator 21 for calculating the gain G1 formultiplication with the corrected image data from the maximum, and amultiplier for multiplying the corrected image data Dout by the gain G1.

In the adaptive gain method, the gain for preventing the overflow ispreferably calculated in frame units.

It is also possible to calculate the gain for every horizontal line andprevent the overflow, for example. This configuration is, however, notpreferable, because the display image looks strange because of thedifference among the gains of the respective horizontal lines.

The above schematically described the fixed gain method and the adaptivegain method.

Inventors confirmed that the amplitude of the corrected image data couldbe suitably adjusted when the gain was calculated by either of themethods.

The present embodiment is thus configured to implement the adjustment ofamplitude by the adaptive gain method.

The following will detail a circuit configuration for implementing theadjustment of amplitude of the corrected image data by the adaptive gainmethod in the present embodiment.

(Maximum Detector 20)

The maximum detector 20 of the present invention is connected to each ofthe units, as shown in FIG. 12.

The maximum detector 20 is a unit of detecting a maximum value out ofthe corrected image data Dout of one frame.

This unit is a circuit that can be readily constructed of a comparatorand a register, for example. This unit is a circuit of comparing thesize of the corrected image data sequentially transferred, with a valuestored in the register, and updating the value of the register with thedata value if the corrected image data is greater than the value of theregister.

The value of the register is cleared to 0 at the head of each frame, andthen a maximum value of the corrected image data in a frame is stored inthe register at the time of the end of that frame.

The maximum value of the corrected image data detected in this way istransferred to the gain calculator 21.

(Gain Calculator)

The gain calculator 21 is a unit of calculating the gain for theadjustment of amplitude so as to keep the corrected image data Doutwithin the input range of the modulator on the basis of the adaptivegain method.

The gain can be successfully determined if it satisfies the followingcondition:gain G≦INMAX/MAX  (Eq 20),where MAX is the maximum value detected by the maximum value detector 20and INMAX the maximum value of the input range of the modulator (firstmethod).

The gain calculator 21 updates the gain in a vertical retrace period tochange the value of the gain every frame.

In the configuration of the image display apparatus according to theembodiment of the present invention, the gain by which the correctedimage data of the current frame is multiplied, is calculated using themaximum value-of the corrected image data of one preceding frame.

Strictly speaking, overflow may occur because of the difference ofcorrected image data between frames accordingly.

To solve this problem, a circuit was designed with a limiter, describedlater, for the output of the multiplier for multiplying the correctedimage data by the gain, so as to keep the output of the multiplierwithin the input range of the modulator, and provided good result.

The above overflow process can be deemed as an overflow process makinguse of the correlation of corrected image data (image data) betweenadjacent frames.

It becomes feasible to prevent the overflow in a configuration without atime delay if a frame memory is provided between the maximum valuedetector and the multiplier.

Inventors confirmed that the gain determining method based on theadaptive gain method could be arranged to calculate the gain by themethod as described below.

Namely, the gain for the corrected image data of the current frame canbe suitably determined so as to satisfy the condition below:gainG1≦INMAX/AMAX  (Eq 21),where AMAX is an average obtained by framewise smoothing (or averaging)of maximums of corrected image data detected in frames preceding to thecurrent frame (second method).

A third method is a method of calculating the gains G1 of the respectiveframes according to Eq 20 and averaging them to obtain the current gain.

Inventors confirmed that all these three methods were suitablyapplicable and that the second and third-methods were exceedinglysuitable, rather than the first method, because they had another effectof largely reducing flicker in the display image (which will bedescribed later with reference to FIG. 16).

Inventors conducted research on the number of frames to be used for theaveraging in the second method and the third method, and confirmed thatacceptable images were obtained with little flicker, for example, byaveraging of 16 to 64 frames.

Just as in the case of the first method, the second and third methodscan decrease the probability of occurrence of overflow because of thecorrelation of (corrected) image data between frames, but also fail toprevent the overflow perfectly.

As countermeasures to it, we employed a method of preventing theoverflow roughly by the above method and preventing the overflowperfectly by provision of the limiter at the output of the multiplier,and obtained better result.

FIG. 16 is a diagram for explaining the flicker, using the first methodand the second method as an example.

FIG. 16 shows an example of a moving picture in which a white bar isrotating counterclockwise over the gray background. In the case of suchan image being displayed, the size of the correction data CD largelyvaries frame by frame with rotation of the bar.

FIG. 17 is a diagram for explaining the corrected image data in thecorrection for such a moving picture. FIG. 17 is a graph of maximums inthe respective frames out of the corrected image data of the respectiveframes.

In FIG. 17 white portions correspond to the original image data, andgray portions to portions extended by the correction.

In the case of the display of the image as shown in FIG. 16, themaximums of the corrected image data in the continuous frames-vary asshown in FIG. 17.

Therefore, if gains are set for the respective frames as indicated by Eq20, the gains of the respective frames will vary heavily as shown inFIG. 18A, so as to intensify the variation of luminance in the displayimage, thereby raising a sense of flicker.

In contrast to it, when the gains are determined by Eq 21, the gains areaveraged and thus the variation of gains becomes smaller, as shown inFIG. 18B, so as to reduce the variation of luminance, thereby achievingthe excellent effect of-reducing a sense of flicker.

In FIG. 18B the plot of white circles represents the gains according toEq 20, while the plot of black circles the gains averaged by Eq 21.

Although the third method was not discussed in detail herein, Inventorsalso confirmed that the variation of gains became smaller, so as toreduce flicker, as in the case of the second method.

It was preferable that the gain calculator 21 should be configured toaverage the gains for the screen of continuous scenes as described abovebut quickly change the gains for the screen of different scenes on theoccasion of switching between scenes of images.

For implementing it, a preferred configuration was such that the gaincalculator 21 was configured to have a preset threshold value as a sceneswitching threshold Gth and calculate the gain of the next frame bysmoothing in such a way that:if ΔG=|GN−GB|>Gth,gain G1=(GN−GB)×A+GB; orif ΔG=|GN−GB|≦Gth,gain G1=(GN−GB)×B+GB

(where A and B are real numbers satisfying the relation of 1≧A≧B≧0),

where GB is a gain of one preceding frame calculated by Eq 20, GN a gaincalculated by Eq 20 from the maximum of the corrected image data of thepreceding frame, detected by the maximum detector 20, and AG an absolutevalue of the difference GN−GB.

Particularly preferable results were obtained when the values of A and Bwere set as A=1 and B= 1/16to 1/64 approximately.

(Multiplier)

The multiplier in FIG. 12 multiplies the corrected image data Dout beingthe output of the adder, by the gain G1 calculated by the gaincalculator, and transfers the result as the corrected image data withthe adjusted amplitude, Dmult, to the limiter.

(Limiter)

If the gain can be determined as described above so as to cause nooverflow, there will arise no problem. It is, however, difficult todetermine the gain with no overflow at all by the several gaindetermining methods described above. Therefore, the apparatus may alsobe provided with a limiter.

The limiter has a preset limit value, compares the output data Dmult fedthereinto, with the limit value, outputs the limit value if the limitvalue is smaller than the output data, and outputs the output data ifthe limit value is greater than the output data (the output is named ascorrected image data Dlim in FIG. 12).

The limiter supplies the corrected image data Dlim, which is completelyconfined in the input range of the modulator, through the shift registerand latch into the modulator.

(Gradation Converter)

Before the detailed description of the operation of the gradationconverter 200 in FIG. 12, a description will be given about a casewherein the image display apparatus is substantiated without correctionfor the influence of the voltage drop and without use of the gradationconverter 200.

Inventors confirmed the following phenomena in the image displayapparatus in the configuration without correction for the influence ofthe voltage drop and without the gradation converter 200.

Namely, the phenomena are as follows:

A. image data with the number of gradation levels being small (darkpicture) provides more reddish display than in the case where a smallregion is displayed in the number of gradation levels (pulse width fordriving) of image data being 255 (in the 8-bit data width);

B. display becomes more reddish in the case of the display of the entirescreen in the number of gradation levels of image data being 255 than inthe case where a small region is displayed in the number of gradationlevels (pulse width for driving) of image data being 255 (in the 8-bitdata width).

Inventors analyzed these phenomena-and found the following reason.

Namely, the red phosphor tended to make its emission efficiencysaturated at the high quantity of injected charge. For this reason,saturation became greater in the case where a small region was displayedin the number of gradation levels of image data being 255. Namely, theemission current is large because of the small voltage drop on thescanning wiring, and thus electrons impinge upon the phosphor over arelatively long period of time, so as to increase the quantity ofinjected charge and thus make the red phosphor saturated. For thisreason, when comparison is made on the basis of the reference in thecase where the small region is displayed in the number of gradationlevels of image data being 255, the quantity of charge injected into thered phosphor is smaller in the above A. and B. cases, so that saturationis less in the red phosphor. This resulted in relatively increasing theintensity of emission of red light, so that the display image becamereddish.

FIG. 19 is a diagram schematically showing the gradation characteristicswithout correction for the voltage drop and without the gradationconverter.

In FIG. 19, the horizontal axis represents the pulse width for drivingof the modulation wiring, and the vertical axis normalized luminanceobtained by normalization with respect to luminances of the respectivecolors in the case where the small region is displayed in the number ofgradation levels of the image data being 255 (in the case of the voltagedrop on the scanning wiring being almost zero). In FIG. 19, a1 gbindicates the gradation characteristic of green and blue in the casewhere the voltage drop in the scanning wiring is almost zero, and a1 rthe gradation characteristic of red in the case where the voltage dropin the scanning wiring is almost zero.

In FIG. 19, c1 gb represents the gradation characteristic of green andblue in the case where the maximum voltage drop occurs with all thedisplay devices on the scanning wiring being turned on, and c1 r thegradation characteristic of red in the case where the maximum voltagedrop occurs with all the display elements on the scanning wiring beingturned on.

The gradation characteristics c1 gb, c1 r were normalized with respectto the luminances in the case where the small region was displayed inthe number of gradation levels of image data being 255. FIG. 19 isillustrated on the assumption that the normalized luminance of green andblue becomes ¼ when the driving pulse width is 255.

In FIG. 19, b1 gb indicates the gradation characteristic of green andblue in the case where the voltage drop occurs so as to present theintermediate luminance between the luminance of a1 gb and the luminanceof c1 gb, and b1 r the gradation characteristic of red at the samevoltage. Similarly, they were normalized with respect to the luminancesin the case where the small region was displayed in the number ofgradation levels of image data being 255.

The characteristics of luminance and driving pulse width (values ofcorrected image data) shown in FIG. 19 vary depending upon voltage dropamounts, and driving voltages of the electron-emitting devices duringdisplay of actual images vary depending upon images and positions ofdevices. It was, therefore, difficult to realize a conversion capable ofcompletely canceling the above characteristics.

Inventors conducted elaborate research and found the following featuresin driving with correction for the influence of the voltage drop of thedisplay panel using the surface conduction electron-emitting devices.

(1) In the method according to the embodiment of the present inventionwith correction for the influence of the voltage drop, data withadjusted pulse widths (corrected image data) is calculated for inputimage data so as to be emitted charge amounts of products of theemission current amounts IE without any voltage drop and the pulsewidths determined by the image data, and the modulator drives thedisplay panel by the pulse widths.

(2) When the maximum of the corrected image data falls outside the inputrange of the modulator, the overflow process is carried out so as tomultiply the corrected image data by the gain. The corrected image datais thus set in the input range of the modulator.

(3) The saturation characteristics of phosphors (in particular, a redphosphor) are characteristics almost determined by emitted chargeamounts in the range of pulse widths and emission current values of theelectron-emitting devices under the actual driving conditions of thedisplay panel.

Namely, the feature (1) indicates that “with correction for theinfluence of the voltage drop, charges enter the phosphors in theemitted charge amounts of the products of the emission currents IEwithout any voltage drop and the pulse widths determined by image data,independent of the voltage drops actually occurring in the display paneland the pulse widths for actual driving” (which means that emittedcharge amount correction in implemented to make correction for variationof-emitted charge amounts so as to achieve the charge amountscorresponding to the image data).

The feature (2) indicates that “with execution of the overflow process,charges enter the phosphors in the emitted charge amounts of theproducts of the emission currents IE without any voltage drop and thepulse widths determined by the values of the image data times the gain.”

Furthermore, the feature (3) indicates that “the saturationcharacteristics of phosphors (in particular, red phosphor) can bedetermined by only the emitted charge amounts.”

Inventors invented the image display apparatus in the configuration withthe gradation converter 200 as a result of deliberation on the abovefeatures (1), (2), and (3).

The gradation conversion characteristics of the gradation converter 200will be briefly described, prior to the description of the actualconfiguration of the gradation converter 200.

In FIG. 20, the horizontal axis represents the emitted charge amountsfrom the surface conduction electron-emitting devices, and the verticalaxis the luminances of the respective colors. For simplifying thedescription in FIG. 20, the emitted charge amounts on the horizontalaxis are presented as normalized quantities with respect to 1 set as acharge amount in the case where an emission current amount IE withoutany voltage drop is injected for only a time Δt equivalent to onegradation level of pulse width modulation. As a result of thenormalization, the maximum of emitted charge amount becomes 255. Namely,when a small area is displayed by the driving pulse width of themodulator of 255 (maximum) (i.e., when the voltage drop on the scanningwiring is almost zero), the emitted charge amount (maximum emittedcharge amount) is 255.

The vertical axis shows the luminances normalized with respect to 1 as aluminance of each color where the emission current amount IE without anyvoltage drop is injected in the case of the pulse width of the 255gradation level (255×Δt).

When the correction is made for the influence of the voltage drop in theembodiment of the present invention, the pulse width is adjusted so thata charge is injected into each phosphor in the emitted charge amount ofthe product of the emission current amount IE without any voltage dropand the pulse width determined by the image data (the feature (1)).

For this reason, in the case of the correction for the influence of thevoltage drop, the horizontal axis corresponds to 0 to 255 of image data.

In FIG. 20 qgb represents the gradation characteristic of green andblue, and qr the gradation characteristic of red. FIG. 20 can beobtained by actual measurement with varying pulse widths or emissioncurrents (driving voltages), for example.

Since the emitted charge amounts are equivalent to the image data in thecase without execution of the overflow process, it is understood thatcorrection should be made so as to effect gradation conversion to cancelthe characteristics of FIG. 20, on the image data. When the gradationconverter 200 is provided with such conversion characteristics as tocancel the gradation characteristics of FIG. 20, it becomes feasible toovercome the aforementioned problem of reddish display.

FIG. 21 shows the actual gradation conversion characteristics forcanceling the characteristics of FIG. 20. FIG. 21 shows a case whereinput and output are 8-bit data. In FIG. 21, QGB represents acharacteristic curve to cancel the saturation characteristic of thegreen and blue phosphors (which is illustrated as a straight line on theassumption that they are not saturated in the present example), and QR acharacteristic curve to cancel the saturation characteristic of the redphosphor indicated by qr in FIG. 20.

Since the image data corresponds to the emitted charge amounts (thefeature (1)), as described previously, the gradation conversion of imagedata made it feasible to cancel the characteristic of the red phosphorhaving the saturation characteristic depending upon emitted chargeamounts

Namely, the gradation conversion of image data means to convert theimage data as luminance requirements to emitted charge amountrequirements taking account of the emission characteristic of thephosphor.

It thus indicates correction for emitted charge amounts to makecorrection for variation of emitted charge amounts toward the emittedcharge amount requirements.

Next described is the case including the overflow process. According tothe aforementioned feature (2), the charge enters each phosphor in theemitted charge amount of the product of the emission current amount IEwithout any voltage drop and the pulse width determined by the value ofthe image data times the gain (factor).

Namely, even if the input image data is the same, the emitted chargeamount with the overflow process is the gain times that without theoverflow process.

For detailed description, FIG. 22 shows the characteristics ofnormalized charge amount versus luminance. In FIG. 22, similar to FIG.20, emitted charge amounts on the horizontal axis indicate normalizedvalues with respect to 1 as a charge amount in the case where theemission current amount IE without any voltage drop is injected for atime Δt equivalent to one gradation level of pulse width modulation. Thevertical axis indicates normalized luminances with respect to 1 as aluminance of each color in the case where the emission current amount IEwithout any voltage drop is injected in the pulse width of the 255gradation level (255×Δt).

The characteristics qgb, qr in FIG. 22 are the same as theaforementioned characteristics in FIG. 20; qgb the gradationcharacteristic of green and blue and qr the gradation characteristic ofred. A square region indicated by GA in FIG. 22 is a region indicatingthe emitted charge amount-luminance with the gain of 1, and thenormalized charge amounts 0 to 255 on the horizontal axis correspond tothe image data 0 to 255 (equivalent to those in the aforementioned casewithout the overflow process).

When the gain is ½, the amount of charge injected into each phosphor isequal to the charge amount of the image data times the gain (½), andthus the image data 0 to 255 corresponds to the normalized chargeamounts 0 to 127. In FIG. 22 a square region indicated by GB is a regionindicating the emitted charge amount-luminance actually obtained whenthe gain is ½.

Similarly, when the gain is ¼, the amount of charge injected into eachphosphor is equal to the charge amount of the image data times the gain(¼), and thus the image data 0 to 255 corresponds to the normalizedcharge amounts 0 to 63. In FIG. 22 a square region indicated by GC is aregion indicating the emitted charge amount-luminance actually obtainedwhen the gain is ¼.

When the gain is G1, the amount of charge injected into each phosphor isequal to the charge amount of the image data times the gain (G1), andthus the image data 0 to 255 corresponds to the normalized chargeamounts 0 to (255×G1). In FIG. 22 a square region indicated by GG is aregion indicating the emitted charge amount-luminance actually obtainedwhen the gain is G1.

As described above, the emitted charge amounts actually achievedcorrespond to values of the product of the image data and the gain (anoperating point determined by the gain).

For this reason, the gradation conversion of image data as describedbelow makes it feasible to cancel the saturation characteristics ofphosphors.

With the gain of 1, the normalized charge amounts 0 to 255 correspond tothe image data 0 to 255, and thus the saturation characteristic of thered phosphor can be canceled out by a γ correction table having theconversion characteristics shown in FIG. 23.

In FIG. 23, QGB indicates a characteristic curve to cancel thesaturation characteristic of the green and blue phosphors (which is astraight line on the assumption that no saturation occurs in the presentexample), and QR (×1) a characteristic curve to cancel the saturationcharacteristic of the red phosphor indicated by qr in FIG. 22.

Likewise, with the gain of ½, the normalized charge amounts 0 to 127correspond to the image data 0 to 255, and thus the saturationcharacteristic of the red phosphor can be canceled by a γ correctiontable having the conversion characteristics shown in FIG. 24.

In FIG. 24, QGB indicates a characteristic curve to cancel thesaturation characteristic of the green and blue phosphors (which is astraight line on the assumption that no saturation-occurs in the presentexample), and QR (×½) a characteristic curve to cancel the saturationcharacteristic of the red phosphor indicated by the GB region of qr inFIG. 22.

When the reference is set in the non-saturated case like QGB, thegradation conversion of image data is to convert the image data (inputdata) 0 to 255 in the range of output data of 0 to 255 (the rangehereinafter will be described on the basis of the reference in thenon-saturated case like QGB).

The output data in the range of 0 to 255 is multiplied by the gainthrough the correction for the influence of the voltage drop, and thenormalized charge amounts of charges injected into the phosphorsthereafter fall in the range of 0 to 127.

The conversion to cancel the saturation characteristics of the phosphorsat the operating point corresponding to the gain is carried out asdescribed above.

In other words, the conversion characteristics to cancel the saturationcharacteristics of the phosphors can be obtained by converting the imagedata (input data) 0 to 255 in the range of output data of 0 to 255,independent of the gain.

Similarly, with the gain of ¼, the normalized charge amounts 0 to 63correspond to the image data 0 to 255, and thus the saturationcharacteristic of the red phosphor can be canceled by a γ correctiontable having the conversion characteristics shown in FIG. 25.

In FIG. 25, QGB indicates a characteristic curve to cancel thesaturation characteristic of the green and blue phosphors (which is astraight line on the assumption that no saturation occurs in the presentexample), and QR (×¼) a characteristic curve to cancel the saturationcharacteristic of the red phosphor indicated by the GC region of qr inFIG. 22.

Similarly, with the gain of G1, the normalized charge amounts 0 to(255×G1) correspond to the image data 0 to 255, and thus the image datais multiplied by the gain (G1) and is converted by the γ correctiontable having the characteristics indicated by QGR and QR (×1) in FIG.23, thereby making correction for the influence of saturation of thephosphors. Furthermore, the output converted by the γ correction tableis multiplied by 1/gain (1/G1) to obtain the output data in the range of0 to-255 for the correction for the voltage drop.

In other words, the above-stated characteristics are equivalent toselection of the gradation conversion characteristics at the operatingpoint determined by the gain.

By the characteristics of the gradation converter 200 as describedabove, it becomes feasible to cancel the aforementioned problem ofreddish display even in the case where the overflow process is carriedout.

A practical configuration of the gradation converter 200 will bedescribed below.

FIG. 26 shows the configuration of the gradation converter 200. In FIG.26 numerals 201 and 203 designate multipliers, 202 a γ correction tablesubstantiated by a memory or the like, and 204 an inverse numbergenerator. This configuration realizes the aforementioned function. FIG.26 shows only the configuration-corresponding to one color, forsimplification. It is a matter of course that the gradation converter200 is comprised of three sets of the same structure for red, green, andblue. In this case, the contents of the γ correction tables are preparedaccording to the saturation characteristics of the phosphors of therespective colors.

The input image data is multiplied by the gain (G1) at the multiplier201. As described previously, the input image data is multiplied by thegain to be converted into the emitted charge amount, and the γcorrection table 202 effects the gradation conversion to cancel thesaturation characteristic of the phosphor as normalized by the maximumemitted charge amount (in the range of 1 to 255).

The γ correction table 202 implements the gradation conversion to cancelthe-saturation characteristic of the phosphor at the emitted chargeamount of charge actually emitted.

Since in this state the output of the γ correction table isgain-multiplied data, the multiplier 203 multiplies the output by 1/gain(1/G1) in order to recover data to be actually subjected to thecorrection for the voltage drop. The inverse number generator 204outputs an inverse number of the gain.

Since the gain is generally smaller than 1, it is necessary to set thebit count of the γ correction table 202 greater than the bit count ofthe image data in order to maintain significant digits, because theimage data times the gain is fed into the γ correction table 202.

The above configuration realized the aforementioned function whereby theaforementioned problem of reddish display was successfully overcome byhardware.

Further, when there is a relation:G=Kg×INMAX/MAX,wherein Kg is a constant and meets:Kg≦1.Then,1/G=MAX/(Kg×INMAX).

Kg×INMAX is a constant,

Kg′ is defined as a new constant:Kg′=1/(Kg×INMAX).Then,1/G=Kg′×MAX.

That is, an inverter 204 can calculate an inverse of a gain bymultiplying a maximum value MAX of the correction image data by theconstant Kg′. In view of the above, the inventor constituted by ROM orthe like can be replaced by a multiplier, thereby reducing hardwarescale and parts.

Further, in case of calculating the second gain (Eq 21), also,similarly, there is a relation:G1=Kg1×INMAX/AMAX,wherein Kg1 is a constant and meets:Kg1≦1.Then,1/G=AMAX/(Kg1×INMAX).

Kg1×INMAX is a constant.

Kg1′ is defined as a new constant:Kg1′=1/(Kg×INMAX).Then,1/G=Kg1′×AMAX.

That is, the inverter 204 can calculate an inverse of a gain bymultiplying AMAX derived by smoothing (averaging) in a frame direction amaximum value of the correction image data detected at a frame prior toa present frame by the constant Kg1′. In view of the above, the inverterconstituted by ROM or the like can be replaced by a multiplier, therebyreducing a hardware scale and parts.

Next, third gain calculating method is performed by calculating the gainaccording to the Eq 20 and averaging. In this case, by means of thecalculation similar to the above second method, the inverter can bereplaced by the multiplier. While, it is necessary to perform theaveraging in both of the gain and the maximum value of the correctionimage data, separately. Thus, the hardware for performing the processingincreases. However, total hardware would be still smaller than thestructure using the inverter.

γ correction table 202 for eliminating the saturation of the phosphormay have following structure.

When the characteristics of γ correction table 202 is such that arequired luminance is Lr and a change quantity to be applied to thephosphor is qr, and the required luminance Lr and the charge quantity qrare both normalized as:qr=fr(Lr),wherein fr (Lr) is a characteristics stored in the γ correction table202 for correcting-the saturation of the phosphor.

And gr (Lr) is defined as a function:gr(Lr)=Lr−fr(Lr).That is, gr (Lr) is a function of a difference of the characteristicscorrelative to the luminance and the charge quantity.

In order to eliminate the saturation of the phosphor, it is necessary tomeet a relation:qr=Lr−gr(Lr).

In the above described embodiments, the γ correction table 202 maycomprise a table of a characteristics gr (Lr) and a subtracter forsubtraction between outputs of tables of characteristics Lr to gr (Lr).In such case, the subtracter should be necessary to constitute hardwarestructure. However, there would be provided an advantage that, in casethat the table of the characteristics gr (Lr) is used for the memoriesof the same capacity, the gradation member can increase, and processingaccuracy would be improved.

FIG. 27 shows another embodiment of the gradation converter. FIG. 27also shows only a configuration for one color, for simplification. Ofcourse, the gradation converter 200 is constructed of three sets of thesame structure for red, green, and blue. In this case, the contents ofthe respective γ correction tables are prepared according to thesaturation characteristics of the phosphors of the respective colors.

In FIG. 27, 202 a, 202 b, and 202 c designate γ correction tables, eachof which stores a conversion table to cancel the saturationcharacteristic of the phosphor corresponding to the emitted chargeamounts of charge actually emitted when the gain is 1, ½, or ¼,respectively. In practice, the conversion tables are equivalent to theaforementioned characteristics shown in FIG. 23, FIG. 24, and FIG. 25.Numeral 205 denotes a linear interpolation unit, which is a unit ofaccepting input of the gain G1, and implementing linear interpolationfrom outputs of two tables on the both sides of the gain G1 out of the γcorrection tables 202 a, 202 b, and 202 c to obtain interpolation valuesfor the gain G1.

Since the characteristic to cancel the saturation characteristic of thephosphor, which is determined by the gain, varies monotonically, theabove configuration permits the conversion characteristic for any gainG1 to be determined by the linear interpolation from the characteristicsof the respective γ correction tables 202 a, 202 b, and 202 c.

As the number of γ correction tables increases, the accuracy becomesmore enhanced, while the cost of hardware rises, naturally. With use ofthree or more γ correction tables, it was feasible to prevent distinctdegradation of display quality.

The above configuration also succeeded in realizing the aforementionedfunction whereby the aforementioned problem of reddish display wassuccessfully overcome by hardware.

Furthermore, it was described in the above description of the presentembodiment that the gradation characteristics of the green and bluephosphors were high in linearity and had no saturation characteristic,but a luminance characteristics of the green and blue phosphors hassaturation characteristic in relation to an electric charge quantityeven though it is much smaller than that of the red phosphor. In thiscase, it is also possible to obtain a normalized gradationcharacteristic as described above for low saturation in each colorphosphor, prepare a table to cancel the characteristic, for each color,and thereby make correction for the saturation characteristic of thephosphor of each color.

Further, the saturation characteristics of the phosphor varies accordingto an acceleration voltage (high power source voltage) between the rearplate and the face plate and to a maximum charge quantity applied to thephosphor. In case of driving a panel, since driving times of respectiveelectron emitting devices are defined, the maximum charge quantity to beapplied to the phosphor is dependent on an emission current IE of theelectron emitting device. i.e., a voltage (Vs) of a scanning unit and avoltage (Vpwm) of a modulating unit. The saturation characteristics ofthe phosphor varies according to the voltage of the high power source,the voltage (Vs) of the scanning unit, and the voltage (Vpwm) of themodulating unit. For an initial adjustment to eliminate performancecharacteristic variation of the display apparatus, and for adjusting byuser, by adjusting the voltage of the high power source, the voltage(Vs) of the scanning unit and the voltage (Vpwm) of the modulating unit,it would be desirable to connect to the γ correction table to cancel thesaturation characteristics of the phosphor for the correspondingvoltage.

(Shift Register and Latch)

The corrected image data Dlim being the output of the limiter isconverted from the serial data format into the parallel image data ID1to IDN for the respective modulation wires by the serial/parallelconversion at the shift register 5, and the parallel image data isoutputted to the latch. The latch latches the data from the shiftregister in accordance with a timing signal Dataload immediately beforea start of one horizontal period. The output from the latch 6 issupplied as parallel image data D1 to DN into the modulator.

In the present embodiment the image data ID1-IDN, D1-DN is 8-bit imagedata. These operations are activated based on the timing control signalsTSFT and Dataload from the timing generator 4 (FIG. 12).

(Details of Modulator)

The parallel image data D1-DN being the output of the latch 6 issupplied into the modulator 8.

The modulator is, as shown in FIG. 28A, a pulse width modulation circuit(PWM circuit) comprised of a PWM counter, and comparators and switches(FETs in the same figure) for the respective modulation wires.

The relation between the image data D1-DN and the output pulse widths ofthe modulator is the linear relation as shown in FIG. 28B.

FIG. 28C shows three examples of output waveforms from the modulator.

In FIG. 28C the top waveform is one for the input data of 0 into themodulator, the middle waveform one for the input data of 128 into themodulator, and the bottom waveform-one for the input data of 255 intothe modulator.

In the present example the bit count of the input data D1-DN into themodulator is 8 bits.

In the above description there were portions describing that themodulation signal of the pulse width equivalent to one horizontalscanning period was outputted for the input data of 255 into themodulator, but, precisely speaking, there are idle periods provided astimewise margins before the rise of the pulse and after the fall of thepulse though they are very short durations, as shown in FIG. 28C.

FIG. 29 is a timing chart showing the operation of the modulator in theembodiment of the present invention.

In the same figure, Hsync designates a horizontal synchronizing signal,Dataload a load signal to the latch 6, D1-DN the-aforementioned inputsignals into the columns 1-N of the aforementioned modulator, Pwmstart asynchronizing clear signal of the PWM counter, and Pwmclk a clock of thePWM counter. XD1-XDN denote outputs of the first to Nth columns of themodulator.

As shown in FIG. 29, with a start of one horizontal scanning period, thelatch 6 starts latching the image data and transferring the data to themodulator.

The PWM counter starts counting on the basis of Pwmstart and Pwmclk, asshown in the same figure, stops counting at the count value of 255, andretains the count of 255.

The comparator for each column compares the count of the PWM counterwith the image data of each column, outputs High during periods in whichthe value of the PWM counter is not less than the image data, andoutputs Low during the other periods.

The output of each comparator is coupled to gates of switches for eachcolumn, and in each Low period of output-of the comparator the upperswitch (Vpwm side) in the same figure is ON and the lower switch (GNDside) OFF, whereby the modulation wire is connected to the voltage Vpwm.

During each High period of output of the comparator contrary, the upperswitch in the same figure is OFF and the lower switch ON, whereby themodulation wire is connected to the GND potential.

When each of the units operates as described above, the pulse widthmodulation signals from the modulator become signals of waveforms with asynchronized rise of pulses, as indicated by XD1, XD2, and XDN in FIG.29.

(Correction Data Calculator)

The correction data calculator is a circuit of calculating thecorrection data for the voltage drop by the aforementioned correctiondata calculation method. The correction data calculator is comprised oftwo blocks of a discrete correction data calculating unit and acorrection data interpolating unit as shown in FIG. 30.

The discrete correction data calculating unit calculates the voltagedrop amounts from the input video signal and discretely calculates thecorrection data from the voltage drop amounts. This calculating unitdiscretely calculates the correction data by adopting the aforementionedconcept of the degenerate model, in order to decrease the calculationamount and hardware scale.

The correction data interpolating unit performs the interpolationoperation from the discretely calculated correction data to calculatethe correction data CD matching the sizes of image data andthe-horizontal display positions x.

(Discrete Correction Data Calculating Unit)

FIGS. 31A and 31B show the discrete correction data calculating unit forcalculating the discrete correction data in the embodiment of thepresent invention.

The discrete correction data calculating unit is, as described below, aunit having a function of grouping the image data into blocks,calculating statistics (numbers of lighting devices) of the respectiveblocks, and calculating time changes of voltage drop amounts at thepositions of the respective nodes from the statistics, a function ofconverting the voltage drop amounts at the respective times intoemission luminance amounts, and a function of integrating the emissionluminance amounts over the time to obtain the total emission luminanceamounts; and calculating the correction data for the reference values ofimage data at the discrete reference points from the total emissionluminance amounts.

In FIGS. 31A and 31B, 100 a-100 d designate lighting device countingunits; 101 a-101 d register groups for storing the numbers of lightingdevices at respective times, for the respective blocks; 102 a CPU; 103 atable memory for storing the parameters aij in Eqs 2 and 3; 104 atemporary register for temporarily storing the-calculation result; 105 aprogram memory which stores programs of CPU; 111 a table memory whichstores the conversion data for conversion of voltage drop amounts intoemission current amounts; 106 a register group for storing the result ofcalculation of the aforementioned discrete correction data.

Each of the lighting device counting units 100 a-100 d is comprised ofcomparators and adders as shown in FIG. 31B. The video signals Ra, Ga,and Ba are fed into the respective comparators 107 a-107 c, to becompared with the value of Cval one by one.

Cval is equivalent to the image data reference value set for the imagedata as described previously.

Each comparator 107 a-c compares the image data with Cval and outputsHigh if the image data is larger, or outputs Low if it is smaller.

The adders 108 and 109 add up the outputs from the comparators, and theadder 110 further adds up the data in every block. The result of theaddition in each block is stored as the number of lighting devices ineach block in the register group 101 a-c.

The lighting device counting units 10 a-d accept their respective inputsof 0, 64, 128, and 192, respectively, as the comparison value Cval ofthe comparators.

As a result, the lighting device counting unit 100 a counts the numberof image data greater than 0 out of the image data and stores the sumsof the respective blocks in the register 101 a.

Likewise, the lighting device counting unit 100 b counts the number ofimage data greater than 64 out of the image data and stores the sums ofthe respective blocks in the register 10 b.

Similarly, the lighting device counting unit 100 c counts the number ofimage data greater than 128 out of the image data and stores the sums ofthe respective blocks in the register 10 c.

Similarly, the lighting device counting unit 100 d counts the number ofimage data greater than 192 out of the image data and stores the sums ofthe respective blocks in the register 101 d.

After completion of counting the lighting devices in the respectiveblocks and at the respective times, the CPU reads the parameter tableaij stored in the table memory 103, as needed, calculates the voltagedrop amounts according to Eqs 2 to 5, and stores the calculation resultsin the temporary register 104.

In the present example the CPU is provided with the sum-of-productsoperation function for smoothly performing the calculation of Eq 2.

The means for implementing the operation according to Eq 2 does notalways have to be the means performing the sum-of-products operation inthe CPU, but it may be substantiated, for example, by storing thecalculation results in a memory.

Namely, it can be contemplated that a memory is configured to acceptinput of the numbers of lighting devices in the respective blocks andstore voltage drop amounts at the respective node positions for allpossible input patterns.

After completion of the calculation of voltage drop amounts, the CPUreads the voltage drop amounts at the respective times and in therespective blocks out of the temporary register 104, makes reference tothe table memory 2 (111) to convert the voltage drop amounts intoemission current amounts, and calculates the discrete correction dataaccording to Eqs 6 to 9.

The discrete correction data thus calculated is stored in the registergroup 106.

(Correction Data Interpolating Unit)

The correction data interpolating unit is a unit for calculating thecorrection data matching the display positions (horizontal positions) ofthe image data and the sizes of image data. This unit performs theinterpolation from the discretely calculated correction data to obtainthe correction data according to the display positions (horizontalpositions) of the image data and the sizes of the image data.

FIG. 32 is a diagram for explaining the correction data interpolatingunit.

In FIG. 32 numeral 123 designates a decoder for determining node numbersn and n+1 of discrete correction data used for the interpolation, fromthe display position (horizontal position) x of image data, and numeral124 a decoder for determining k and k+1 in Eq 17 to Eq 19 from the sizeof the image data.

Selectors 125 to 128 are selectors for selecting discrete correctiondata and supplying it to linear approximation units.

Numerals 121 to 123 denote linear approximation units for effecting thelinear approximations of Eq 17 to Eq 19, respectively.

FIG. 33 shows a configuration example of the linear approximation unit121. As seen from the operators in Eq 17 to Eq 19, the linearapproximation unit can be generally comprised of subtracters,accumulators, an adder, and a divider, for example.

However, the linear approximation unit is preferably configured-so thatthe number of column wires between nodes for calculation of the discretecorrection data and intervals of image data reference values forcalculation of the discrete correction data (i.e., time intervals forcalculation of voltage drop) are powers of 2, because it can provide theadvantage that the hardware can be constructed in very simple structure.When those are set in powers of 2, (Xn+1−Xn) in the divider shown inFIG. 33 becomes a value of a power of 2, which can be implemented simplyby a bit shift.

If the value of (Xn+1−Xn) is always a constant value and value expressedby the power of 2, the output of the divider can be obtained by shiftingthe addition result of the adder by the degree equivalent to the power,and, therefore, the divider does not have to be provided.

In the other portions, when the intervals of the nodes for calculationof discrete correction data and the intervals of the image data arepowers of 2, it becomes feasible to readily fabricate the decoders 123,124, for example, and to replace the operations in the subtracters inFIG. 33 with easy bit operations, thus providing many merits.

(Operation Timing of Each Unit)

FIGS. 34A to 34C present a timing chart of operation timing of eachunit.

In FIGS. 34A to 34C, Hsync denotes a horizontal synchronizing signal;DotCLK a clock created from the horizontal synchronizing signal Hsync bythe PLL circuit in the timing generator; R, G, and B digital image datafrom the input switching circuit; Data image data after the datasequence conversion; Dlim the output of the limiter, which is thecorrected image data after the correction for the voltage drop; TSFT ashift clock for transferring the corrected image data Dlim to the shiftregister 5; Dataload a load pulse signal for latching the data in thelatch 6; Pwmstart a start signal of the aforementioned pulse widthmodulation; the modulation signal XD1 an example of the pulse widthmodulation signal supplied to the modulation wiring 1.

With a start of one horizontal period, the digital image data RGB istransferred from the selector 23. Let R_I, G_I, and B_I be the inputimage data during the horizontal scanning period I in FIGS. 34A to 34C.Then those image data items are stored during one horizontal period inthe data sequence converter 9, and are outputted as digital image dataData_I in alignment with the pixel array of the display panel during thehorizontal scanning period I+1.

The image data items R_I, G_I, and B_I are fed into the correction datacalculator during the horizontal scanning period I. This calculatorcounts the aforementioned lighting devices and obtains the voltage dropamounts at the end of the counting.

Subsequent to the calculation of voltage drop amounts, the discretecorrection data is calculated and the calculation result is stored inthe register.

When the time moves into the scanning period I+1, the data sequenceconverter outputs the image data Data_I in the preceding horizontalscanning period, and in synchronism therewith, the correction datainterpolation unit performs the interpolation from the discretecorrection data to calculate the correction data. The correction dataafter the interpolation is supplied to the adder 12.

The adder 12 sequentially adds the correction data CD to the image dataData and transfers the corrected image data Dlim after the correction tothe shift register. The shift register stores the corrected image dataDlim of one horizontal period according to Tsft and effectsserial-parallel conversion thereon to output parallel image data ID1-IDNto the latch 6. The latch 6 latches the parallel image data ID1-IDN fromthe shift register according to a rise of Dataload, and transfers thelatched image data D1-DN to the pulse width modulator 8.

The pulse width modulator 8 outputs the pulse width modulation signalsof the pulse widths according to the latched image data. In the imagedisplay apparatus of the present embodiment, as a result, the pulsewidth modulation signals outputted from the modulator are displayed witha delay of two horizontal scanning periods behind the input image data.

Display of images was actually carried out using the image displayapparatus as-described above, and it was verified that it was feasibleto make the correction for the voltage drop amounts on the scanningwiring, which was the problem heretofore, to make improvement in thedegradation-of display images caused thereby, and to provide display ofremarkably excellent images.

The adoption of the several approximations presented the exceptionaleffects of facilitating the proper calculation of correction amounts ofimage data for the correction for the voltage drop, implementing it byvery simple hardware, and so on.

Second Embodiment

The corrected image data Dout is the result of the addition of imagedata Data and correction data CD.

Unless the result of this addition falls within the input range of themodulator, the correction can induce overflow, which raised the concernthat another sense of strangeness appeared in the display image.

In order to solve this problem, the first embodiment was configured toprevent the overflow by the method of detecting the maximum of correctedimage data, calculating the gain so that the maximum becamecorresponding to the maximum of the input range of the modulator, andmultiplying the corrected image data by the gain.

As compared therewith, the present embodiment is similar in thedetection of the maximum of the corrected image data, but is differentin that the size of the image data before execution of the correction islimited so that the aforementioned maximum becomes corresponding to themaximum of the input range of the modulator.

Namely, in order to prevent the overflow, the input image data ispreliminarily multiplied by a gain to narrow the amplitude rangethereof, thereby preventing the overflow.

The overflow process of the present embodiment will be describedhereinafter with reference to FIG. 35.

In FIG. 35, 22R, 22G, and 22B denote multipliers; 9 a data sequenceconverter; 5 a shift register for one line of-image data; 6 a latch forone line of image data; 8 a pulse width modulator for outputtingmodulation signals into the modulation wires of the display panel; 12 anadder; 14 a correction data calculator; 20 a maximum value detector(unit) for detecting the maximum of corrected image data Dout in aframe; 21 a gain calculator; 200 a gradation converter. The gradationconverter 200 will be described later, and the following descriptionwill be given on the assumption that the modulation converter 200 isabsent.

R, G, and B denote parallel RGB input video data; Ra, Ga, and Baparallel RGB video data after the inverse γ conversion process; Rx, Gx,and Bx image data resulting from the multiplication by GAIN G2 at themultipliers; GAIN G2 a gain calculated by the gain calculator; Dataimage data after the parallel-serial conversion at the data sequenceconverter; CD correction data calculated by the correction datacalculator; Dout image data corrected by the addition of correction dataand image data at the adder (corrected image data); Dlim corrected imagedata obtained by limiting Dout below the upper limit of the input rangeof the modulator, by the limiter.

(Multipliers 22R, 22G, 22B)

The multipliers 22R, 22G, and 22B are units for multiplying the imagedata Ra, Ga, Ba after the inverse γ conversion by GAIN G2.

More specifically, each of the multipliers multiplies the image data byGAIN G2 according to the gain determined by the gain calculator andoutputs the image data Rx, Gx, Bx after the multiplication.

GAIN G2 is a value that is calculate by the gain calculator and that isdetermined so that the corrected image data Dout, which is the result ofthe addition of the image data Data and the correction data at the adderdescribed hereinafter, falls within the input range of the modulator.

(Maximum Detector 20)

The maximum detector 20 will be described below.

The maximum detector in the embodiment of the present invention iscoupled to each of the units, as shown in FIG. 35.

The maximum detector is a unit of detecting a maximum value out of thecorrected image data Dout of one frame.

This detector is a circuit that can be readily constructed of acomparator and a register, for example. This detector is a circuit ofcomparing the size of corrected image data Dout sequentially transferredthereto, with the value stored in the register and updating the value ofthe register with the data value if the corrected image data Dout isgreater than the register value.

The value of the register is cleared to 0 at the head of a frame, andthen a maximum MAX of the corrected image data in that frame is storedin the register at the time of the end of the frame.

The maximum MAX of the corrected image data detected in this way istransferred to the gain calculator.

(Gain Calculator)

The gain calculator is a unit of calculating the gain so thatthe-corrected image data Dout falls within the input range of themodulator, with reference to the detected value MAX of the maximumdetector. In the present embodiment, the gain calculator also calculatesthe gain for adjustment of the amplitude of the corrected image data onthe basis of the adaptive gain method.

Alternatively, the gain may be calculated by the fixed gain method, fromthe viewpoint of preventing the overflow of the corrected image data inthe configuration of the present embodiment (FIG. 35).

The gain determining method can be a method of determining the gain soas to satisfy the following condition:GAIN G2≦(INMAX/MAX)×GB  (Eq 22),where MAX is the maximum of corrected image data Dout in one frame,INMAX the maximum of the input range of the modulator, and GB GAIN G2calculated for an immediately preceding frame by the gain calculator.

This gain calculator updates the gain in a vertical trace period andchanges the value of the gain every frame.

In the configuration of the image display apparatus according to theembodiment of the present invention, the gain by which the correctedimage data of the current frame is multiplied is calculated using themaximum of the corrected image data of the preceding frame.

Namely, the apparatus is configured to prevent the overflow by makinguse of the correlation of corrected image data (image data) betweenframes.

Strictly speaking, overflow can occur because of the difference ofcorrected image data between frames accordingly.

In order to solve this problem, the circuit was designed with a limiterlocated at the output of the multiplier for multiplying the correctedimage data by the gain so that the output of the multiplier always fellinto the input range of the modulator, and it presented better result.

Inventors confirmed that the gain could be calculated by another methodas described below, in addition to the above gain determining method.

Namely, the gain for the corrected image data of the current frame canbe determined so as to satisfy the following condition:GAIN G2≦(INMAX/AMAX)×GB  (Eq 23),where AMAX is an average obtained by averaging maximums of correctedimage data detected in frames preceding to the current frame.

In the above condition GB denotes GAIN G2 calculated for the immediatelypreceding frame by the gain calculator.

Another method can be a method of calculating GAIN G2 for each frameaccording to Eq 22 and averaging gains of respective frames to obtainthe current gain.

Inventors confirmed that any of these three methods was suitablyapplicable in terms of the prevention of overflow and concluded that thegain was preferably calculated by the method of Eq 23 in considerationof occurrence of flicker as described in the first embodiment.

Inventors conducted research on the number of frames to be used for theaveraging of maximums of corrected image data in the gain calculationmethod of Eq 23 and found a preferred configuration of averaging themaximums of corrected image data in the range of 16 to 64 framespreceding to the current frame, with better result.

It is needless to mention that the present method is configured, morepreferably as shown-in FIG. 35, so as to prevent the overflow completelyby the limiter for limiting the output of the adder.

The gain calculation method may also be modified based on detection ofthe scene change, as in the first embodiment.

(Gradation Converter)

In the second embodiment, much the same phenomena as in the firstembodiment were recognized in the absence of the gradation converter200.

The second embodiment is different only in the place for themultiplication by the gain in the overflow process, and is thus providedwith the gradation converter 200 in the configuration similar to that inthe first embodiment. The characteristics and configuration of thegradation converter are the characteristics of FIG. 22, FIG. 23, FIG.24, and FIG. 25 and the configuration of FIG. 26 or FIG. 27, as in thefirst embodiment. This configuration successfully canceled the influenceof saturation of phosphors and overcame the aforementioned problem ofreddish display.

When the configuration of the gradation connecter 200 is one shown inFIG. 26, as shown in the first embodiment, a table of a characteristicsof gr (Lr) being a function of a difference between characteristicscorrelative to a luminance and a charge quantity, and a subtracter forsubtracting between outputs from tables of the characteristics Lr and gr(Lr) may be used.

And, when the configuration of the gradation converter 200 in the secondembodiment is the configuration shown in FIG. 26, the multipliers 22R,22G, 22B in FIG. 35 and the multiplier 203 and the inverse numbergenerator 204 in FIG. 26 are omissible. The reason is that the inputdata into the multiplier 203 is multiplied by 1/gain in the multiplier203 and the output therefrom is further multiplied by the gain at eachmultiplier 22R, 22G, 22B, so that the input data into the multiplier 203is equal to the output data from the multipliers 22R, 22G, 22B.

The configuration in this arrangement is presented in FIG. 37 and FIG.38. Since the structure and operation are the same, the descriptionthereof is omitted herein.

Furthermore, just as in the case of the first embodiment, it was alsodescribed in the second embodiment that the gradation characteristics ofthe green and blue phosphors were high in linearity and had nosaturation characteristic, but in practice a luminance characteristicsof the green and blue phosphors has a saturation characteristic inrelation to an electric charge quantity, even though it is much smallerthan that of the red phosphor. In this case, it is also possible toemploy a method of determining the aforementioned normalized gradationcharacteristic for low saturation in each color, preparing a table tocancel the characteristic, for each color, and thereby making correctionfor the saturation characteristic of the phosphor of each color.

It is also possible to downsize the hardware scale in such a way thatthe characteristics of the γ tables 202 a, 202 b, 202 c in FIG. 27 aredetermined in consideration of the characteristics of the inverse the γprocessor 17 and the inverse γ processor 17 is excluded.

As described in the first embodiment, the saturation characteristics ofthe phosphor varies according to on acceleration voltage (a voltage ofthe high power source) between the face plate and the near plate and toa maximum charge quantity applied to the phosphor. In driving the panel,since the driving times of respective electron emitting devices aredetermined, a maximum charge quantity to be applied to the phosphor isdependent on an emission current IE of the electron emitting device,i.e., a voltage. (Vs) of the scanning unit and a voltage (Vpwm) of themodulation unit. The saturation characteristics of the phosphor variesaccording to the voltage of the high power source, the voltage (Vs) ofthe scanning unit and the voltage (Vpwm) of the modulating unit. For aninitial adjustment for eliminating performance characteristic variationsof the display apparatus, and for user's adjustment, in case of varyingthe voltage of the high power source, the voltage (Vs) of the scanningunit and the voltage (Vpwm) of the modulating unit, it is desirable toconvert into γ correction table eliminating the saturationcharacteristic variations of the phosphor corresponding voltage.

Furthermore, in the case of the image display apparatus according to theembodiment of the present invention, with entry of nonzero, uniformimage data common to all the colors, the apparatus is driven with theprocess of canceling the influence of the voltage drop so that a pulsewidth of a pulse from the modulator close to the output terminals of thescanning circuit becomes shorter than a pulse width of a pulse from themodulator far from the output terminals of the scanning circuit.

Furthermore, as a result of canceling the saturation characteristics ofphosphors dependent upon the emitted charge amounts of theelectron-emitting devices, the driving is implemented without deviationof luminance balance among displayed colors, i.e., at almost even colortemperature of white color, for any image data uniform and common to allthe colors.

The embodiment of the present invention presented the example of makingthe correction to cancel the saturation characteristics of thephosphors, and it is noted that the same configuration as in theembodiment of the present invention can also make correction for changeof gradation characteristics due to the electron emission amountsbecause of the influence of degradation of the driving voltage waveformsfor the electron-emitting devices (waveform rounding) or the like.

As described above, the image display apparatus of the present inventionsucceeded in making proper improvement in the degradation of displayimage due to the voltage drop on the scanning wiring, which was theproblem heretofore.

The adoption of the several approximations provided the remarkablyexcellent effects of facilitating the proper calculation of correctedimage data with correction for the influence of the voltage drop,implementing it by very simple hardware, and so on.

Furthermore, the image display apparatus of the present invention wasprovided with the overflow processing circuit for preventing theoverflow of the image data after the correction from the input range ofthe modulator, whereby the overflow was prevented by the gain.

Since the gradation converter for changing the gradation conversioncharacteristics according to the gain was provided in the stagepreceding to the configuration of making the correction for theinfluence of the voltage drop, it was feasible to successfully cancelthe saturation characteristics of the phosphors and thereby display theimages with high quality.

1. An image display apparatus comprising: a plurality ofelectron-emitting devices connected to a plurality of row wires andcolumn wires respectively and arranged in a matrix pattern; a phosphoremitting light in response to an irradiation with an electron emittedfrom the electron-emitting devices; scanning means connected to said rowwires and applying a selection voltage to selected ones of said rowwires; modulating means connected to said column wires; gradationconverting means for performing gradation conversion of input image dataaccording to a characteristic of the phosphor; and corrected image datacalculating means for calculating corrected image data, which is imagedata after correction for influence on a voltage applied to anelectron-emitting device due to a voltage drop through said row wires,corresponding to the converted image data outputted from the gradationconverting means, wherein the modulating means inputs the correctedimage data, and outputs modulation signals to said column wires, and acharacteristic of the gradation converting means is set so as to correctthe light emission characteristics in a case of no voltage drop.
 2. Animage display apparatus comprising: a plurality of electron-emittingdevices connected to a plurality of row wires and column wiresrespectively and arranged in a matrix pattern; a phosphor emitting lightin response to an irradiation with an electron emitted from theelectron-emitting devices; scanning means connected to said row wiresand applying a selection voltage to selected ones of said row wires;modulating means connected to said column wires; gradation convertingmeans for performing gradation conversion of input image data accordingto a characteristic of the phosphor; corrected image data calculatingmeans for calculating corrected image data, which is image data aftercorrection for influence on a voltage applied to an electron-emittingdevice due to a voltage drop through said row wires, corresponding tothe converted image data outputted from the gradation converting means;and amplitude adjusting means having a multiplier for multiplying by afactor the corrected image data of a value beyond an input range of themodulating means so as to be changed into a value falling within aninput range of said modulating means, wherein said gradation convertingmeans has a gradation conversion characteristic corresponding to thefactor, and said modulating means inputs the corrected image dataamplitude-adjusted by said amplitude adjusting means, and outputsmodulation signals to said column wires.
 3. An image display apparatusaccording to claim 2, wherein the gradation converting means comprises amultiplier or a plurality of correction tables for performing aconversion to cancel saturation characteristics of phosphors at aplurality of operation points determined by the factor.
 4. The imagedisplay apparatus according to claim 3, wherein said gradationconverting means is comprised of a multiplier for multiplying image databy said factor, and a γ correction table of canceling saturationcharacteristic of the phosphor in an absent state of the voltage drop,and is configured to feed an output of said multiplier into a γcorrection table of canceling a gradation characteristic of luminance inthe absent state of said voltage drop.
 5. An image display apparatusaccording to claim 3, wherein the gradation converting means comprises aplurality of gamma correction tables corresponding to a plurality of thefactors of different values.
 6. The image display apparatus according toclaim 3, wherein the characteristic of said gradation converting meansis a characteristic of canceling greater saturation of a phosphor whenthe factor is larger than when the factor is smaller.
 7. The imagedisplay apparatus according to claim 2, wherein said amplitude adjustingmeans adjusts the amplitude of the corrected image data outputted fromthe corrected image data calculating means, by multiplying input imagedata before the correction, being an input into the corrected image datacalculating means, by a factor for adjusting the amplitude thereof. 8.The image display apparatus according to claim 2, wherein said amplitudeadjusting means detects a maximum of outputs of said corrected imagedata calculating means in each frame and adaptively calculates thefactor so that the maximum matches an upper limit of the input range ofsaid modulating means.
 9. The image display apparatus according to claim2, wherein the factor is a factor preliminarily determined so that withentry of maximum input image data an output of said corrected image datacalculating means does not overflow the input range of said modulatingmeans.
 10. The image display apparatus according to claim 2, whereinsaid corrected image data calculating means comprises: means forestimating a spatial distribution and a temporal change of voltage dropamounts to be caused on the row wires during one horizontal scanningperiod, corresponding to input image data; and means for calculatingcorrected image data with correction for the input image data, from thevoltage drop amounts calculated.
 11. The image display apparatusaccording to claim 2, wherein said corrected image data calculatingmeans comprises: means for discretely estimating spatial distributionsand temporal changes of voltage drop amounts to be caused on the rowwires during one horizontal scanning period, corresponding to inputimage data; and means for calculating corrected image data withcorrection for said input image data, from the voltage drop amountscalculated.
 12. The image display apparatus according to claim 2,wherein said corrected image data calculating means comprises: means fordiscretely estimating voltage drop amounts to be caused on the row wiresduring one horizontal scanning period, in a spatial direction and in atemporal direction, corresponding to input image data; discretecorrected image data calculating means for discretely calculatingcorrected image data for image data corresponding to times of thecalculation of said voltage drop amounts at spatial positions of thecalculation of said voltage drop amounts, from said voltage dropamounts; and corrected image data interpolating means for performinginterpolation between outputs of the discrete corrected image datacalculating means to calculate corrected image data matching sizes andhorizontal display positions of the input image data.
 13. The imagedisplay apparatus according to claim 2, wherein said modulating means isa pulse width modulation means for implementing modulation by varying apulse width of a voltage pulse waveform applied to each column wire,according to an input into the modulating means.
 14. The image displayapparatus according to claim 2, wherein said gradation converting meanshas a function of converting input image data into an emitted chargeamount requirement to cancel a saturation characteristic of a phosphor,and outputting the emitted charge amount requirement, and wherein saidcorrected image data calculating means has a function of makingcorrection for variation of an emitted charge amount due to influence ofsaid voltage drop, for the emitted charge amount requirement being anoutput of said gradation converting means.
 15. The image displayapparatus according to claim 2, wherein the corrected image datacalculated by said corrected image data calculating means is adjusted sothat said emitted charge amount requirement becomes an emitted chargeamount in an absent state of the voltage drop to be caused a row wire.16. An image display apparatus comprising: a plurality ofelectron-emitting devices connected to a plurality of row wires andcolumn wires respectively and arranged in a matrix pattern; a phosphoremitting light in response to an irradiation with an electron emittedfrom the electron-emitting devices; scanning means connected to said rowwires and applying a selection voltage to selected ones of said rowwires; modulating means connected to said column wires; gradationconverting means for performing gradation conversion of input image dataaccording to a characteristic of the phosphor; corrected image datacalculating means for calculating corrected image data, which is imagedata after correction for influence on a voltage applied to theelectron-emitting devices due to a voltage drop through said row wires,corresponding to converted image data outputted from the gradationconverting means; and amplitude adjusting means having a multiplier formultiplying by a factor corrected image data of a value beyond an inputrange of the modulating means so as to be changed into a value fallingwithin an input range of the modulating means, wherein said gradationconverting means has a gradation conversion characteristic correspondingto the factor, and said modulating means inputs the corrected image dataamplitude-adjusted by the amplitude adjusting means, and outputsmodulation signals to said column wires, and wherein, responsive to aninput of non-zero uniform image data common to all colors, a pulse widthof a pulse outputted from the modulation means in a vicinity of anoutput terminal of the scanning means is made smaller than a pulse widthof a pulse outputted from the modulation means distant from the outputterminal of the scanning means, and a saturation characteristic ofphosphors being dependent on a quantity of a charge discharged from theelectron-emitting devices is cancelled, so as to implement such drivingthat any image data uniform and common to all the colors is displayed atalmost equal color temperature of white color, independent of emissionluminance.
 17. An image display apparatus comprising: a plurality offirst wirings to which a selected potential is applied; a plurality ofsecond wirings to which a modulation signal is supplied;electron-emitting devices connected to the first and second wirings; aphosphor emitting light in response to an irradiation with an electronemitted from the electron-emitting devices; a modulating circuit forsupplying a modulation signal to the second wirings; a conversioncircuit for conversion corresponding to a characteristic of thephosphor; and a multiplier circuit for multiplying a gain of data of avalue beyond an input range of the modulating circuit outputted from theconversion circuit to the modulating circuit, so as to be changed into avalue falling within the input range of the modulating circuit, whereinthe conversion circuit performs conversion corresponding to the gain.18. An image display apparatus according to claim 17, wherein theconversion circuit comprises a multiplier circuit for multiplying inputdata by the gain.
 19. An image display apparatus according to claim 17,wherein the conversion circuit has correction tables capable ofoutputting respectively different conversion values correspondingly togains of plural values for one input data.
 20. An image displayapparatus according to claim 19, wherein the conversion circuit outputsa conversion value in an interpolation manner.
 21. An image displayapparatus according to claim 17, wherein the conversion circuitcomprises a correction circuit for correcting at least a voltage dropthrough a first wiring, and the correction circuit inputs data from theconversion circuit, and outputs corrected data to the multipliercircuit.
 22. An image display apparatus according to claim 17, furthercomprising a determination circuit for determining the gain based on avalue of the data inputted from the converting circuit.